Caves and crypts do not respawn anything internally except the yellow mushrooms. The Black Forest crypts will respawn one external guard after the initial 3 are killed. Both caves and crypts are limited to campfire, sitting log and cooking station as far as things you can put in them. They make a good resting spot if it's night and you're not ready for nocturnal mobs. The external stones they are made of are indestructible and some players integrate them into a base. Use your imagination for some cool creations.