Castelo de São Jorge, situated at the top of a hill in the Alfama neighbourhood of Lisbon is well worth the 'hike' up. I believe you can take the tourist tram 28 up to the top if you don't fancy the walk but Rob and I enjoyed all the pretty streets and you feel fantastic (if a little sweaty) when you do get to the top. As we were on a pretty tight budget we didn't pay for many activities during our time in Lisbon (food takes priority!) but the castle accepts student cards and we were pleased we managed to fit a visit in.
It is mostly just a ruin but a very large ruin at that and fantastically preserved. The entrance fee of 8.50 euros (5 for concessions) is absolutely worth it with the most spectacular view over the skyline and river Tagus.
Have any of you guys been to Lisbon? What was something you couldn't miss?