In my opinion, Karlstejn is a nice getaway, not too far from Prague. It will give you the chance to get out of the capital, but without getting too far away. The opportunity to discover the countryside if you're only spending a few days in the Czech Republic...
On the way there, you’ll quickly see the Gothic castle that dominates the landscape. You'll feel like you're in another time. You'll have to climb a little (on foot) to reach the building, taking a road lined with shops and restaurants, and especially some pretty facades. The inside of the castle is just as impressive as the outside!
When you leave by car, I recommend stopping at a restaurant on the way back - more authentic than in Karlstejn - but especially to visit the Nenacovice glassworks, to discover the glassblowing profession and to bring back a small glass item as a souvenir.
'Charles Castle', Karlstejn is one of the most visited Czech monuments for a reason: it's a magnificent castle taken directly from a fairy tale, placed at the top of a hill which towers over the surrounding landscape so typical of Bohemia (small green hills and forests as far as the eye can see). However, despite the obvious interest of this fortress dating from the 14th century, be aware that you won't be the only ones to visit the place during your stay in the Czech Republic...
The journey from the station to the castle seems more like a procession through the countless shops selling souvenirs and other fake medieval items which, in my opinion, considerably ruin the place's charm. The only solution to my liking in order to really appreciate the place is to arrive from the other side of the castle as mentioned in my pieces of advice.