I have to throw something else in about absolutely needing to book some sights ahead of time.

I went to London around the 1st of April and wanted to just wing it with my sightseeing itinerary because I was using my 4 nights in London as a time to recover from jetlag before a pretty intensive and physical tour of Jordan. Everyone said absolutely have a schedule set up with all your main sights booked ahead. I didn't do that because I wanted to have flexibility depending on what I felt like doing and at what time.

Every single place I went I just strolled in without reservations and never had to wait. Westminster Abbey (and I got a verger tour), Churchill War Rooms, a special exhibit at the National Gallery, and Windsor Castle on a Sunday. I was in busy, but not peak, season just like you'll be.

Although I understand you might not wish to commit to tours in advance, what's the advantage of arriving at any destination without advance hotel reservations?
Am I misunderstanding something? Are you changing your itinerary at the last minute?
Even with that in mind, a reservation with 24h advance cancellation should be sufficient.

It sounds interesting and adventurous "winging" it but somewhat reckless. It's easy to book a hotel a day or two before so you can see if it has the amenities, you might need like private bathroom vs shared, pool or spa if needed and breakfast included which for me is a nice perk. As for transportation, Uber is available in Portugal and reliable but you must have the EXACT address of the location you are going to since the hotel we stayed at which was the Curio Collection by Hilton in Cascais had one address listed on its website and one that the Uber driver needed so he wouldn't drive by and drop you off a block away.

I usually have my itinerary planned as to stops so would never not have hotel reservations. Long time ago when young I did that, not anymore, and I did waste lots of time when I did do that and got some lousy places. Guess what is left last minute? Now this does depend somewhat on season, of course, In some places in popular tourist seasons, you can't do that if you have a particular area you want to stay.

Now once I did do this about five years ago in France. That was a change of plans where I was alone and kind of freaked out about renting a car to do my original plan. Yes, I had rented a car before but this time I had to drive out of the center of a large city, and I only drive AT, so was worried they might not have what I ordered available. In any case, I had a backup plan of an itinerary I could do solely by train and had done some research on where I'd stay if I pivoted to that plan. That was one of my favorite trips actually, because I was not going to some of the top tourist cities, so I had no trouble getting my reservations by booking online a couple days in advance. Same for train tickets, of course. But I didn't totally wing it, I had an idea of where I"d stay. I did not just show up in a town without any reservations.

I don't think anyone walks around with their luggage looking for a hotel, at least I hope not. Even my parents, who were not savvy travelers at all on their first trip to Europe asked the TI office or a taxi driver for a decent budget hotel and got one. They didn't wander around with luggage (thank goodness).

My one problem with one of the suggestions is the idea that you will be in a hotel and then just decide to stay as long as you want (the idea that what if you like it, you don't want to be forced to leave). I don't know where that is where you can do that, anytime you make reservations you have to put a departure date. You can't just decide on a day's whim whether to leave or not. Even some liberal hotels where you can cancel without penalty up to 24 hrs in advance don't usually allow that. Unless they have room of course, and vacancies. But then that would mess up your plans if you are doing this by train as you wouldn't know if you needed a train ticket or not if you just get up and ask can you stay.

So if this trip is without a car and has some general plan, yes, you can research in advance possible hotels, and know where you want to book online in advance. That's what I did, worked perfectly. I think I had top two choices on my list. So I didn't spend a lot of time when I did book them, I knew exacty what hotels to check for vacancies. I just did it night before in my hotel room with my tablet, which I travel with.

what mjs, kja, maitaitom, madam, Christina, Michael and other veteran Spain travelers have told you.
The days of "winging it" are really, simply over, especially with the massive amounts of tourism this year in Spain, almost to reach the French levels....unless one travels in the winter months to places where there are no winter celebrations/fiestas and no foreign tourists, which isn't the case in most itineraries.

And I live here in Spain and know very well from experience where reservations are necessary or not (in un-touristed areas), yet I never travel without advanced reservations anymore, simply because I don't wish to waste my travel hours searching for a hotel in an unknown city. I never, ever travel in this country without prior hotel reservations. I've learned that lesson.

For each specific country or region or season, I can't possibly tell you. But for some places, those days are definitely not over. Not everyone likes to be locked in for every day, and there are places where it is not essential. It depends.

If you want to test out my theory, just go onto Google Maps, pick a place in Portugal for tomorrow and see what is available. What is over: the days of guesswork. No longer any need to go driving around a city in the hopes of finding a hotel. It's all there at your fingertips.

I do know Portugal and do know what cities are now considered "over touristed" and what cities are yet "under the radar".
But... tourism in Portugal is at its highest level ever this year, a very, very different "landscape" from the past, (as in the case of Spain, which may overcome the tourist numbers in France this year).
I would personally never want to "wing it" in Porto, Lisbon, Sintra, Cascais, the Algarve in the high season, even Evora (although I would avoid the hugely hot summer) or even in Madeira or in the Azores, where we were recently, although of course, it's "possible" if one isn't particularly about one's' lodging or the location of said lodging.

My point is that I personally don't want to waste my travel time searching for a hotel in an unknown city especially if I haven't done research and I don't know the best locations in that city and ease to public transportation or walkability etc.

In the major cities of Portugal and Spain, with over (55+) years of Iberian travel experience both in Portugal and Spain, I simply no longer "wing it" because I have experienced the downsides first hand. I never, ever "wing it" any more, simply because we travel on the Iberian Peninsua monthly all over the peninsula and its islands, since we now live here and realize the penalties of "winging it" in the most heavily touristed areas of Portugal and Spain.

Clearly, you want to book in advance, and that's totally fine for you. That is mostly what I do.

But the OP asked, based on a YT video. The OP is already eliminating Lisbon, Sevilla, Porto, Coimbra, etc from this strategy. I am simply confirming that the OP can travel that way if it is their preference.

We've found that Europe in general is just a very different place post-Covid, much more crowded in most places (both city and countryside). "High season" seems to start earlier and extend later every year, and we no longer arrive anywhere (by car or otherwise) without a lodging reservation for at least the first night or two. We're older and not as willing to laugh off the occasional strange/disappointing/ghastly hotel experience or pay significantly more than we want to just to feel comfortable about our choice. We're in the no-bad-surprises-if-we-can-help-it years...

This thread reminds me of the time we drove from Zurich, , through Italy and south of France , Loire Valley ,ending in Paris.
All before the internet, and the euro . In August.
I remember parking in front of a hotel, walking in and asking for a room . It worked well , until Monte Carlo..
Anyway, it was a zillion years ago ..
Now, I don’t move without a reservation ( free cancellation )😳

No doubt, if you know exactly where you are going and on which dates, it is smarter to book in advance. If nothing else, you will probably pay less for the same room.

However, like the OP, not everyone wants to be locked in all the time with a rigid itinerary. That is what the OP was asking about.

As far as ghastly experiences go, just pick a town in Portugal and see what is available for tomorrow. I am pretty sure you can avoid ghastly experiences. Nowadays, it is far easier than ever to get what you want. In fact, it is march harder to nail down a hostel with multiple beds in a room and shared bathrooms at the last moment. Those places are in short supply and get snagged quickly.

I gave no advice to the OP. I just expressed our post-Covid observations of crowds and our travel preferences, developed after decades of experience.

Everyone travels differently, some more risk averse than others. We don't know what type or rating of accommodation the OP would hope to find at the last minute, how flexible the budget is, etc.

The Azores Archipelago and the interior of Portugal are not under tourist pressure. It's true that everyone wants to visit Lisbon, the Algarve, and Porto, but the reality is that the best hospitality, history, and gastronomy are found in the interior of Portugal.

I prefer having accommodations chosen at least a day in advance as I don't like spending a lot of time searching for an available spot. When we used to travel without reservations, we would feel like we had to start searching for a place early in the afternoon rather than having that time for sightseeing, a late lunch, etc.

I know very well the feeling of trying to dig up a place to stay at the last second, pre-internet.

Now, you can so easily find out exactly which places have availability, see extensive photos, room amenities, location, and ratings. It's amazing how little time it takes.

Or, depending on one's interests and preferences and other constraints (cost; distance to sites of interest; with parking, elevator, breakfast, pool, a/c .. or not to any or all of those desires), how much time it takes would, I think, would vary.

Seriously, have we not established that there are pros and cons and that people will differ in how to weigh them?

I apologize if I haven’t made myself clear. In my post I was not talking about pros or cons. I was simply correcting some misinformation.

It is easy to book at the last minute in many places in Portugal. Want a pool? You can filter for that on hotel booking websites.

Gone are days when you had to search around blindly and waste valuable time.

Furthermore, anyone who doubts this can go online now and look at availability for tonight and you will see you have many options with all the specifics at your fingertips.

I have been surprised several times traveling with very high hotel rates due to something going on in the area. Last year I was surprised by Boston hotel rates of $800-$1000 a night for hotels like the Marriott. Got hit with something like that in Singapore with the Grand Prix. Also I am particular about the quality, location and value of lodgings. You usually get less options if booking last moment. If you do not care about such things then yes it is highly likely you can find some place to stay last minute.

Many hotel offer cancellation (yes, higher rates) up to 24 hours of arrival. My preferred option .