I recommend the Actun Tunichil Muknal cave trip. Your kids will love it! You can do Ian Andersons cave tubing as a day trip, but the cruise-ships are offering that tour. So it can be very crowded. The Barton Creek cave tour is great too. You don't have to book tours a head of time though. You may want to just take a walk around SI and see what the tour guides are offering before you decide.