What kinds of food do you like to eat? And what are your interests? Prague has a bit of everything, you could take a look at the scrollback (stuff that's been said before you came in) for suggestions I have given Angel :)
As well, the art museum on the other side of the river (after you cross over the Charles Bridge) is a great experience. They have the penguins on the water lit up at night. If you have time, go to Prague 2 where there's a fortress (Vyšehrad) that's really beautiful. Most of it is free to go to as well, minus the museum which costs like 50 kr
Angel: Prague is absolutely amazing, definitely take some time to go to the church next to the Charles Bridge, the oldest organ in the Czech Republic (and I believe Europe) is played every night at 20.00. A beautiful experience if you like music. As well, you can get a bite to eat under the Charles Bridge next to the water, if you go through the passageway that leads to the dance club next to the torture museum there's a restaurant about halfway in. It used to be owned by a terrible owner, but the new owner has really revamped it and it's so beautiful. The food is really good as well, and decently priced for the city location. It's usually quiet in there as well because the restaurant had such a bad rep back then. It's called the Charles Bridge restaurant now.