OK so cave tubing isn't zip-lining on the thrill factor scale. But it is a very enjoyable activity nonetheless. I liked the walk into the jungle when the guides showed us many interesting things along the way. The cool clear water is lovely after the hot hike.
Good point, about staying closeby. Make an overnight or two in the area (spend that money opn the hotel rather than the transportation costs.) Then you can see some Mayan ruins while you're there. San Ignacio, or Benque. We liked the Mopan River Resort, although (I think) they have a 3 night minimum, and close at certain times of the year. They have a website if you are interested.