Yes Ceske Krumlov is a bit far for a day trip especially if you are leaving that evening ,other suggestions are good. I do like Kutna Hora as there is plenty to do there for a day and only an hour or so from Prague.
Karlstejn is also an easy trip but if just doing the castle then it really is only a half day. I will be there next month with a small walking group. Going to visit the castle then do the walk from there to Beroun and get the train back to Prague .
Plzen is great especially if you want to visit the Brewery .Tabor is also a nice place to visit about 90 minutes by train from Prague.You could also do Terezin about an hour from Prague by bus or the village of Lidice that was destroyed after the Heydrich assassination, beautiful memorial there and a very interesting museum,but again it is only really a half day trip, bus from Nadarzi Veleslavin takes about 20 minutes or so.