While I could spend an unlimited amount of time in Prague, I'll try to help a bit...
Around CZ: Česky Krumlov, Plseň and Dresden are all good day trips, although I'd say that Dresden is worth 2 days/1 night and Česky Krumlov is worth an overnight as well.
Dresden and Plseň are easy to get to by train, the last one is probably best by bus.