Fish, fish, fish. The crab is amazing. You get a hammer and hammer your way through delicious fish meat.
The famous fish restaurant 'Ramiro' is a couple hundred meters down the street. It has lines for hours, not worth it! Walk up to this one with no line, half of the price and frankly better food as they don't need to rush.
In the very touristic Graça area, there is this heaven place, hidden from tourists. This café belongs to the theater da Garagem. It has an amazing view, the atmosphere is peaceful and with a lot of light. They don't have wifi if you plan to work there though..
Saturdays are the perfect days to visit Chiado and look for a rare or second-hand book calmly exhibited on the booksellers’ stalls all day long. From 10 am to 5 pm, a bit of Portuguese history can be found here, from comic books to recent releases, illustrated postcards and special editions.