On the train to Porto now and thought I would note some things from our Lisbon experience over the last few days:
-Overall, things were not too crowded which was great and the weather was perfect!
-COVID status: We made copies of our vaccine cards, but no one has asked to see them anywhere yet. Masks are still required on all forms of public transit, taxis/Ubers, and in any museums/culture sites/palaces (people will come reprimand you right away if you remove your mask). In shops and restaurants, waiters/staff wear masks and patrons are a mix. Cloth masks that everyone wears in the US aren't really worn (I'm sure there's no problem with it but you would definitely stand out).
-Sintra: Great day trip, we went on a Saturday and crowds were nothing like I imagined! Do as Rick says though and charge your train/metro card the day before. The self service machines in Rossio wouldn't accept any of our credit/debit cards or cash and the line in person was long so we almost missed the train. We bought our online ticket to Pena once we were on the train. We took a taxi straight to Pena Palace which cost about 11 euros and took 15ish minutes. Note that the Lakes entrance of Pena is closed. From there, we walked to Castle of the Moors and back into town (we took the less scenic way down that Rick mentions -- took us about 20 mins but it is steep). Quinta Regaleira was a short easy walk from town. The only line we really experienced in Sintra was to get into the Initiation Well there. And bring a jacket, it can be windy and quite a bit colder than in Lisbon.
-No lines at all at Pastéis de Belém, which was good
and worth a visit but we liked Manteigaria better.
-Restaurants were pretty busy on weekend nights so if you're set on a place, try to make a reservation.
-Cervejaria Trindade (beer hall on the Bairro Alto walk) is closed temporarily. We got drinks from the kiosk in Largo do Carmo instead which was a nice place to sit in the square for a while.
-Wear comfortable shoes, it is quite hilly! I love to walk so we walked almost everywhere (with a few taxis/Ubers to locations outside the center). We actually didn't end up taking any trams/metro/buses because most things were within a 20 minute walk.
Let me know if you have questions!