Having just booked air tickets we have 3 extra nights to add to itinerary. Traveling with my husband and no car. Comfortable with trains, Uber, buses etc. Perhaps a private driver in certain areas. We are active, retired and adventuresome. Like walking everywhere, gardens, natural beauty i.e. coastline, and seeing the sights but staying away from too touristy locations as much as possible. Here is our current itinerary. Any suggestions on how to plan these three days. Guimares? Tavira? Longer where we're already planning to go? Thank you so much for your suggestions!
Current Itinerary with number of nights. 25 days.
Lisbon 4
Sintra 2
Cascais 3
Porto 2
Douro Valley 4
Coimbra 2
Evora 3
Lagos 4
Lisbon - last night before departure on 5/10

We did four days in Douro Valley, and although it worked out well in the end, I would probably split that 2 and 2 with Guimarães. Porto was our favorite town. You cold easily add a day or preferably 2 there. We also loved Tomar, which is not your list. Good luck.

Great suggestions. Thank you. Did you go out to the Foz when you stayed in Porto?

We did not go out to Foz. I have a trip reported started here on Fodor's. At the rate I'm going, hopefully it will be done before you leave There are plenty of more here on Fodor's (some who went to Foz I believe) so you should have a wealth of info at your hands.

1889mdmd,
A few notes...
If you plan not to rent a car, then for the Douro Valley you would want to hire a taxi or driver-guide, especially with 4 nights, if you plan any winery touring, as most of the top quintas are spread throughout the valley. There are 4 in Pinhão, but they can be seen easily in 2 days. We do spend 4 nights in the valley but focused on extensive wine touring (& business related).

The valley is pretty in late May but not so much in early to mid April, when the landscape can be quite brown. If you plan to do extensive hiking, you would also need a car to reach the trailheads, or hire a guide. https://dourowalks.com

I would normally agree with maitaitom to split your 4 nights, with 2 in the valley and 2 in Guimarães, a UNESCO World Heritage City, and the "gem" of the Minho, but to reach it from the Douro you would best use a transfer. The train from Porto to Régua or Pinhão works well, but to reach other cities from the Douro by public transportation is time consuming/complicated.

For Sintra, with the current very heavy crowds, (please read maitaitom's wonderful, ongoing TR!) you might want to spend 3 nights there (booking if possible at the charming Chalet Saudade) to see all the monuments at a less hurried pace and 2 nights in Cascais. Use Uber to move from Sintra to Cascais (the crowded Carris bus 1623 wouldn't be comfortable with luggage)

What are your particular plans for the Algarve? The attractions are golf, beaches (cold water!), water sports (weather dependent) and hiking. Some locations mid-Algarve are very touristy. Long beachside walks and boat trips (again, weather dependent) you would also have in Cascais.
From Evora to Lagos you'd need to take the Rede Expressos bus with 1 transfer. The ride takes 4 hrs. 30 min. Would it be worth it to you?

Your time in Porto seems short, as it's been a favorite of many Fodorites who have traveled there this year (read informative TRs by progol and Melnq8 as well). I would add 2 days.
When in Porto we do always go out to upmarket Foz on a sunny day for a coffee or wine at a beach bar, great beachside walks along its Atlantic promenade and to Matosinhos for a fabulous grilled seafood feast. But we have 4 nights.

And I agree also with maitaitom about Tomar if you have any interest in Manueline architecture and the history of the Knights Templar. It's just a pleasant town to spend a couple of nights in and handy for visits to Alcobaça or Batalha, but with a driver/guide.
Again, there's another but....... you would need to reach Tomar by bus or train, and both may require a change, depending on your departure city.

From Coimbra to Evora your best option is the Rede Expressos bus with 1 direct bus a day at 3:30 pm, as there would be 2 train changes.

Just a few thoughts to get you started.

Bus schedules: https://rede-expressos.pt/en (Portugal's national bus carrier)
Train: cp.pt

Maribel - thanks for your input. Taking your advice and added a third night in Sintra and booked at Chalet Saudade. Thinking of deleting the Algarve from the itinerary as the weather at that time is not that great and we really want to linger where we go elsewhere. Any hotel suggestions in Porto? Will definitely extend time there to 3 or 4 nights per suggestion from maitaitomi and you!

Hi 1889mdmd,
So glad that you booked at the Chalet Saudade. Several of our Fodorites have stayed there recently and have been delighted with it and with the hospitality of the hostess. I'm sure you'll enjoy it as well.

Yes, the weather at that time in the Algarve isn't optimal.

In Porto there are many lovely hotels in the Aliados, Rua das Flores, Largo de São Domingos area in the center of the city.
It does depend on your budget, but I would look at:
Porto Bay Flores,
the new-ish Casa da Companhia
or the Pousada on the Rua das Flores (these 3 get top reviews on booking.com)

or...
New on Aliados, the Editory Boulevard Aliados.
Le Monumental Maison Albar at Aliados is simply gorgeous but pricey, a luxury property.
The Mercure Porto Centro Aliados is off the square and new and will be much less expensive

The Infante Sagres is a beautifully restored city icon (the oldest hotel, I believe), one of my favorites for its gorgeous Belle Epoque interior and classic service.
It was recently sold by Taylor Fladgate to a new owner, Gas Capital, so check recent reviews.