Visit Karlstejn Castle
Prague was the capital of the Holy Roman Empire not once but twice, from 1346 to 1437 and then again in the late 15th century. Karlstejn Castle was initially built to house and guard the Crown Jewels of the Empire. A great little day trip outside of Prague, this magnificent castle is a a mere 40 minute train ride away.
A round trip ticket should cost you 99cz and trains depart just about every 30 minutes. Keep your eyes open on the train, the Czech Republic has some gorgeous country side to gaze at as you zoom by.