I actually was looking into this not long ago, how to make things interesting in caves... my main reference here was Cave Story, especially the puffy enemies that can hover for a while... they are passive, and get an aggressive behaviour when on range, hover then try to smash the player when above. The bats wait then dive toward the player when in range, then go slowly back to sleeping position if they miss.
I like the idea of idle enemies that will not be aggressive if you don't disturb them, with a different behaviour depending of situation (attack, flee, taunt, jump ledge, dive from ledge...), it gives a feeling that the caves are having a life/routine of their own, that you just disturbed, and will slowly get back to peace once you leave.