Too late for this trip, but I have to suggest another place to visit in Slovenia, especially for anyone even remotely interested in natural wonders - Postojnska Jama (Postojna Cave).
It's one of the largest / accessible caves in the world (that I'm aware off) and definitely worth a visit. It's located about 50km SSW of Ljubljana.

Some info:
http://www.postojnska-jama.si/
http://www.showcaves.com/english/si/.../Postojna.html