We (husband and myself) are considering 2+ weeks in September to tour Portugal by train.
Flying in and out of Lisbon from US.

A tentative itinerary I’ve come up with -

Lisbon, 5 nights
Porto, 5 nights
Coimbra, 3 nights
Cascais, 3 nights (side trip to Sintra)

We’d book a ride to Lisbon airport from Cascais at the trip’s end, as I understand it is a 30 minute drive (?).
Or should we return to Lisbon proper, or outside the airport for the last night?

Are 3 nights each in either Coimbra or Cascais too much time, or not enough? Our time is flexible, but for various reasons, we prefer to not go over 2 and 1/2 weeks.

Thinking of mid-September to beginning of October because it’s convenient for us, and guessing the weather might still be warm and pleasant. Fwiw, we’re not interested in swimming or sunbathing at Cascais, just want to see the town and the coast there, I’ve read it’s quite pleasant and worth staying overnight.

I have other questions about train travel there, but I plan to research that after booking flights.

Thanks very much in advance for any replies, suggestions and advice.

I like your plan to spend 5 nights in Lisbon. It's a very interesting city with lots to see and do. And interesting neighborhoods to explore. And many miradouros (viewpoints) where you can sit, have a drink and enjoy the scenery. We had 5 nights in Lisbon but that included a daytrip to Sintra, so I wish we had an extra night for Lisbon.

We took a bus tour to Sintra (which we don't usually do) and that tour included a drive and brief stops in Cascais. My husband and I both commented that Cascais seems like a lovely place to spend a night or two to relax and enjoy the coastal scenery. So I think spending a few nights there will be nice at the end of your trip.

Regarding Sintra, I think it would be nice to spend a night or two in Sintra instead of seeing it as a daytrip. I wish we had done that. There are lots of palaces and castles to see, and Sintra is very busy and very crowded so I think it would be more enjoyable to be there in the evenings and early mornings when the daytrippers have left. So perhaps take a night from Cascais and maybe Porto so that you can spend a night or two in Sintra.

I haven't been to Porto or Coimbra so can't really comment on your allocation of nights.

Have you thought about purchasing open-jaw (multi-city) airline tickets? Fly into Lisbon and home from Porto so that you don't have to backtrack.

Thanks for your insights, KarenWoo.
Re the open-jaw - I am considering that, too. I'm actually booking award flights, so I'll see if I can get something practical from Chicago and back.

As for Sintra, I was thinking I would take the bus from Cascais on one or two days. I am not married to staying in Cascais, and I've considered staying in Sintra instead. But it does really seem like there may be enough to see and do to stay in either town for at least 3 nights.
I've also considered staying in Cascais at the very beginning of the trip, then heading to Lisbon 2nd, then heading north toward Porto, and finally flying out of Porto.

Thanks again!

Booked our flights tonight! No sense in waiting… So, we are going now, for sure.
16 nights. Flying into Lisbon and out of Porto.

For Coimbra: we had two nights and that worked out great. This gave us a full day to do all that we wanted in the city, and a little more time the evening of arrival and morning of departure to wander a little more. We did not do other touring in that specific area. We drove there from Pinhao (Douro Valley) and then went on to Lisbon. What we easily fit into a day in Coimbra: the university tour, the Museu Nacional Machado do Castro, Fado o Centro, plus wandering, eating, shopping! We did not end up doing the botanical garden. Even if you see Fado in Lisbon, i'd recommend also going here because it is its own type, and you learn alot from the show.

I'd take away a night from Coimbra and spend it in Guimaraes the night before you leave. It's an easy trip by train from Porto and then to the airport by bus or taxi. It's a magical, historical city with a lot to do, wander and experience. I think it's worth spending the night there. I recommend the Hotel da Oliveira, right on one of the main squares.

Otherwise, maybe add a night to Porto and doing day trips such as to the Douro Valley and Guimaraes, the Penedes National Park (we took a wonderful day tour), among other options. Or, just keep your three nights in Coimbra, and figure out what you want to explore beyond the town itself with that as a base.

I like your idea of spending eight nights generally in the Lisbon area (and i like that you divide it between Lisbon / Cascais or Lisbon / Sintra, because even though both are easily reachable for the day from Lisbon, there are some benefits to staying overnight in those towns.).

Personally I would only spend two nights in Coimbra and spend a night in Sintra. We only stopped at Coimbra for an afteroon quick visit and we did Sintra as a day trip tour from Lisbon that also included Cascais. Sintra is lovely and I am sorry we didn't have more time there. We spent 3 nights in Porto including a drive to Guimares. On the way as we were traveling by car we visited Aviero for a canal boat ride and Baltalha to visit the monastery. We also visited the monastery at Alboaca and I enjoyed both better than the very crowded monastery in Lisbon. We spent one night in Batalha and so we visited the monastery first thing in the morning and we practically had the whole place to ourselves. One of our favorite places in Portugal however was Evora and we spent two nights there.

Skatedancer and Madam397, Thank you both for your very helpful replies!

I may take a night away from Coimbra then, and just spend 2.
We can either add a night to Cascais (or Sintra) or add a night to the Porto leg.

Thanks especially for suggesting Guimares. That was not on my radar before, and I’ll look into it.