Rzeszow is not the prettiest of cities but nearby is Lancut castle (it`s pronounced something like wan-sewt), well worth a visit. It is 17 km from Rzeszow and if memory serves, there is a fairly convenient bus service.

I agree with Andrzej that Krosno is a very pretty city - also, if you have some time to wander around, Sandomierz, Zamosc and (in the Tatra mountains) Zakopane.