Ruined castles are my favorite, especially when they still rather look like castle and have little nooks and crannies you can explore. Choustnik fits the bill. Even cooler, the castle's main tower still stands strong, and you can climb up for great views.
Then if you follow the green trail a little ways around the castle, you'll come to some cool rock formations you can climb around on. Instead of keeping to the trail, follow the rocks for as long as you want along this natural playground. Danger: between the castle and these perfect forest-ambush areas, you might start feeling like you've strayed into a Robin Hood movie.
If you go...
To get to Choustnik Castle, take one of the infrequent buses from Tabor or other nearby towns to the village of Choustnik. Be sure to check bus schedules for the return trip. The castle is a pleasant walk along the green trail, just over a kilometer away.
Entry fee: currently 40 Czech Crowns for adults, 20 for children
Open: July and August--every day. May, June, September, and October--only weekends and holidays.