In my humble opinion, caves can be very interesting. Here, a couple ideas on top of my head:
*Crystal formations: see these:
https://www.google.co.uk/search?q=cryst ... 20&bih=971
*In-cave lakes, ponds, rivers and waterfalls
*bats
*stalagmites and stalagtites
*echoing sounds, howling winds
*luminescent mushrooms that glow in the dark
*larger than normal insects
*bones of dead animals, fossils
*caveman paintings from a long past era
*remains of a lost civilisation
*cave bear!
*snakes.
*holes above that cast rays of light from sun
*holes below that glow red because of a near-surface lava lake
*exhuming gas from such holes
*ice formations
*in-cave plants (which have surprisingly vivid colors like violet, orange, azure etc.)
Also, even basic rock formations and textures can be very interesting. here, check this out:
http://listverse.com/2013/11/18/10-most ... d-revised/
Hope that helps <img src="{SMILIES_PATH}/icon_e_smile.gif" alt=":)" title="Smile">