Hmm... Katowice is definitely worst choice, there is simply nothing to look at besides industrial areas
Other options:
Berlin: 4:40 to 5:20 away, tickets from 29 euro one way
Dresden: 2:15 away from Prague, tickets from 22 euro for one day return or 26 euro one month return
Munich: from 6 hours by direct train or 5 hours train+railway bus, tickets from 29 euro one way (Bavaria-Bohemia ticket might be an option in certain cases)
Wroclav(?): from 5:12 with at least one train change, return ticket - 42 euro, but can be don cheaper with a small tariff trick.
Katowice might be substituted by Krakow, but it is already more than 8 hours away.
Additionally, we would also recommend to consider Alpine Austria (cities and/or nature), Leipzig and probably some small historic towns in Bavaria region.