Super cool cave with a lake inside of it! It's not anything terribly exciting but it's beautiful…read moreand hearing the history of its formation is worth a visit. It was discovered in 1943 after an earthquake, and there's another set of caves behind it which aren't accessible by the boats that you ride in to look at this one.
The tour is pretty short, like 20 minutes, but the guides are fun/silly and you can ask them anything.
There's a bar in the waiting area (also a gift shop with lots of doo dads) where you can get a nice beer before your boat trip.
It's only 10 CHF so pretty affordable!