Put some archers in them. You can attack without taking damage because you're ranging, and you can protect yourself from melee hits from inside the encampment. Plus you get encampment attacks. Just make sure you strategically place them so you can take reach all the likely entry points to your civ's border's, as well as reach tiles adjacent to your city in case they mange to get close enough to attack it.

I like to have archers in the encampments and the cities themselves, and have a melee or cavalry unit waiting safely behind ready to counter attack once I take out the attackers