The Timeout Food Market in Lisbon is definitely worth a visit one evening.
Pink street and Bario Alto have a lot of cool bars.
In terms of site seeing I think it's definitely worth paying to get into the castle (great views up there), I wasn't as fussed with the elevator thing in the centre but it does have good views.
If the weather is good the beach which is 2 or 3 stops from Lisbon on the train is worth a visit too.