Bohemia is compact, and several of the best-known day trips from Prague are genuinely short journeys rather than expeditions.

Kutná Hora is the one most worth the effort: a silver-mining town with a cathedral that rivals anything in Prague, plus the ossuary that most people actually come for. It is about an hour by train.

Karlštejn is the castle closest to the city and the busiest; Český Krumlov is the most beautiful and much too far for a comfortable day return, whatever the tours claim — it is better as an overnight.

Articles here do the arithmetic: what each trip costs, which train, how long it really takes, and which are worth doing at all.

Two practical notes that apply to all of them. Buy the train ticket at the station or in the operator's app rather than from an agency in the old town, where the same journey is routinely marked up. And check the last return service before you go — several of these towns are small enough that the evening timetable thins out sharply, and being stranded in Kutná Hora is a more expensive mistake than the fare you saved.