Absolutely Aviero! I may be an oddball here but I really liked Aviero. It’s a real Portuguese town without hoards of tourists that is so easy to enjoy. Yes, ride the canal boats. They’re wonderful but don’t forget to translate the sayings and pictures painted on the bows of each one. They are delightfully naughty and raunch. Enjoy the beautiful Art Noveau architecture. Visit the fish market in the morning where you’ll be out numbered by locals 20 to 1. Go shopping and have lunch with the locals at their shopping mall along the canal. Visit the salt flats. Spend the day at the beach at Costa Nova enjoying the 19th century fisherman’s cottages painted delightful colors. Eat Ovo Moles in the city it was invented. Hell yes, Aviero!
I’ve experienced Sintra twice. Once as a day trip from Lisbon. I felt like part of the tourist hoard even when I arrived very early. It didn’t warm me. The second time, I spent a couple nights in Sintra and fell in love with it. It’s a different vibe after the tourist masses leave. I really enjoyed it. I still think Lisbon itself deserves a few nights but I highly recommend doing Sintra as an overnight.
Anyway, that’s my two cents. Enjoy Portugal!