Take your time
Lisbon is a huge city with vast options. Take your time, when exploring it! Don't miss out of Fado & fish. Also check out the different beaches and a trip to the neighbouring city of Sintra, also called the fairytale town. I planned to stay for three days, but added another day while being there.
Insider Tip:During 3 days each year, the underground caves are opened for visitors. Go very early, or you'll wait forever in the queue. If you don't manage to get in, go to the local H&M store. It has a glass floor that allows glimpses down to the caverns. ;) Check out for travel passes to save money, and also for combi tickets for the museums when going to Sintra.