We just got back from a few days in Lisbon/Sintra. Weather sounds awful there right now, but hopefully these tips are still valuable:
1) The RS self-guided walks in Lisbon are all fantastic, and you should do all of them. But the three walks take up a full day. You may want to do Alfama stroll (starting with the Castle - I wouldn't "skip" it!) and Baixa stroll on one day, and then Barrio Alto/Chiado stroll on another day, so that you can do all the little side things/museums that RS recommends.
2) When doing the Alafama walk, definitely get lunch at Miss Can! Great place!
3) You must do dinner (get the 30 euro/person tasting menu at A Taberna do Mar in Graca.)
4) Time Out Market was also fantastic for food.
5) A great option for shopping if you're interested is to focus on Chiado and check out Isto (all clothes made sustainably in Portugal -- especially good for casual menswear); dishware at Cerâmicas na Linha (R. Capelo 16 -- I think it's their outlet location?), and more made in Portugal housewares/clothing at Burel Mountain Originals (two locations, right next to one another).
Happy to field more questions!
Best,
Dave