http://www.naturalbridgecaverns.com/...%29/Index.aspx is very well developed and on the way to Gruene, New Braunfels, or even Austin.
http://www.cavewithoutaname.com/ is also an excellent cavern and has a very nice 'off-the-beaten-path' feel. It could also work its way into a day trip to Fredericksburg http://www.fredericksburgtexas-online.com/