What to buy:
If you're looking for a gift for children to bring home from your travels in Slovenia, you could do a lot worse than to buy the charming contemporary fairy tale "Jami the dragon". Jami is a dragon who comes into conflict with a greedy goblin and is ultimately transformed into an olm (the salamander that lives in the caves). It is well illustrated and is available in Slovenian, English, German and (I speak under correction) French. I think that fairytales - with their themes of goods vs evil - have a universal appeal, and for a child that has been on the cave tour, they will be able to relate to the underground environment in which the story is set. This is available in a number of the gift shops at the cave entrance at Postojna.
Otherwise, the tourist shops stock a good range of the usual tourist goodies, with an emphasis on semi-precious stones and fossils.
Postojna is the most visited cave in Slovenia so it's the most touristy that you'll visit. Near the ticket booth, there are gift shops where you can get the usual assortment of tshirts, refrigerator magnets, postcards and a variety of other things that you just can't live without.
There's also a place to get snacks, a sit down restaurant and even a hotel.
What to buy: We bought David a trinket for his international trinket collection, a little stone Proteus Anguinus which is the sightless colorless "human fish" that you find inside the Caves. We also bought a bottle of FerMedica for my sister in law who was watching my dog and cat, a honey and brandy drink in a pretty designer bottle.