Porto and Lisbon day trips

Porto: Braga (Bom Jesus) and Guimarães (birth place of Portugal). I would mention a few more places in the Minho region, but without a car it would be a bit more difficult to visit.

Lisbon: Evora and if you are into ancient history, take a tour of the megaliths.

https://www.eboramegalithica.com/?lang=en

I dislike crowds but love Sintra. The main area that is crowded is if you want to enter Pena Palace. Personally not my favorite palace in Sintra anyways. You can view the grounds of Pena (Park of Pena ticket), Quinta da Regaleira (really fun place to explore with the well and tunnels), National Palace of Sintra (right in town and liked the rooms), Monserrate (amazing gardens, not crowded and my favorite), and the Moorish Castle.

https://www.parquesdesintra.pt/en/parks-monuments/

Just my opinion. We enjoyed all your suggested day trips except Aveiro - found that underwhelming. I thought it was billed as the Venice of Portugal. Umm, no.

Skip Aviero. Prioritize Guimares.

I too was underwhelmed with Aveiro in September. There are a couple canals with the typical tourist boat ride, but not much else. It didn’t help that a large portion of the historic district along the canal was a huge construction zone.

Óbidos is worth a couple hours to walk along the wall and walk by the castle, but the main street through town is jammed full of day trippers until about 7pm.

We thoroughly enjoyed Coimbra and I feel it deserves a couple full days.

Other possible places to visit could include Batalha and Alcobaca monasteries, Fátima, and Tomar’s Convent of Christ. We had a rental car so I can’t speak to reaching anyplace using public transportation.

I spent a few hours in Braga and it is wonderful. Check out the Cathedral of Braga and The Sanctuary of Our Lady of Sameiro.

In Lisbon, check into Belem as well as Sintra. I was there late October and it was still very crowded. Lisbon is going to be very crowded in May no matter what and so is Sintra but Sintra is worth it.

Two very short train trips from Lisbon are Cascais and Sintra. Both have plenty to see and are very different from one another and from Lisbon itself. We spent 2 nights in each and also stayed a couple of nights in Belem. In Sintra we beat most of the crowd by taking a tuk-tuk up to Pena Palace before the day trippers could arrive from Lisbon and beating the public busses from town by a half hour or so. We had planned to take the bus from Sintra to Cascais but Uber was so inexpensive in Portugal that we just called for a car.

If you are at all interested in wine, take a day tour of the Douro valley. It’s easiest by organized small group tour. There are many. Gaia - across the river from Porto is fun - again if you’re interested in wine. Matosinhos is a nice metro or bus ride from Porto for great a great fish lunch. They grill them outside and the aroma in the area is great. The fish is wonderful, too.
Obidos is a bit difficult by train and, as mentioned, is overrun with tourists. I’d skip it. I agree with being underwhelmed by Aveiro. Coimbrã is great and worth your time. Conimbriga, outside of town is very nice, also.

Evora would be my preference over Sintra, simply because of the crowds. I don’t go over there (Sintra) anymore for that reason. However, you can visit Montserrat without all the crowds and it’s lovely. You really won’t have time for many day trips with just a week in Lisbon.

What I personally think is a pleasant day in Sintra in Spring without getting overwhelmed with crowds would be..
1st thing in morning Quinta da Regaleira (can walk from the train station). Mostly outdoors and the two places where there are lines are the Initiation Well and Palace itself. Go to the Initiation Well early then view the rest of the grounds. Next, catch a bus to Monserrate Palace (there is one that leaves from Regaleira so no need to return to town). Explore the wonderful gardens etc. then head back to town via bus or tuk tuk or uber/taxi. Depending on your energy level, visit the National Palace of Sintra that is right in town at the end of the day. Then head back by train to Lisbon.

You can get lunch at Regaleira or bring sandwiches with you to eat on the lawn at Monserrate by the Palace. Lots of families relaxing on that lawn. Peaceful place.

Thank you, fellow travelers. I knew I could count on you!
Porto - we will scratch Aveiro, which seemed to be your unanimous decision ;-). I love the idea of the Douro wine tour, but alas, my husband doesn’t drink wine so… I will look into Guimaraes. It wasn’t even on my radar!
Lisbon - one morning in Belem and a day in Sintra, but skip the interior of Peña Palace
Driving from Lisbon to Lagos - Evora and the megaliths
Thank you again for your help!