We just took a student group to Barton Creek Cave yesterday, and we all had an amazing time canoeing through the cave. They had more than enough canoes for our group of 30, and our guide Carlos was very knowledgeable and safety conscious too. The cave tour goes in around a kilometer, and you get to see all sorts of cave formations: stalagtites, stalagmites, cloumns, flowstone, etc.
You'll get a mini history lesson, and pick up some neat facts about Mayan culture and religion. They used to perform sacrifices in the cave, and there is a skull that you can see from around 20 feet away. All in all, it's a great half day trip!
If you get to the Barton Creek Outpost - http://www.bartoncreekoutpost.com/ - they have ice cold beers. They also have a great swimming hole with rope swings and hammocks to hang out in. If you get in at Mike's Place, be sure and take some pictures with Mango, their spider monkey.