If you're doing a whirlwind tour of Slovenia, Postonja Caves should probably be first on the list. The tour is an amazing dive into one of the largest subterranean areas in all of Europe.
Photos can be a little difficult though, as they don't allow flash, so take advantage of any well-lit sections.
When we travel, we're not big spenders on tours, but this one was worth the price of admission. We skipped the extra visit to the animal area, because with the basic tour, you still get to see the famed albino salamander that is native to the cave.