In Belize we did the cave tubing and it was wonderful. We took our then 6 year old, then 15 year old and a bunch of grownups and other than the hike to get to the the entry point it was a lot of fun. The hike is not intended for those with bad knees, hips or ankles, just an FYI.
In Honduras we hired a cab at the pier to do an island tour, for $10 per person he took us to a wonderful beach for a couple hours and then about an hour on tour. We had a nice easy day, which we needed after a couple of other very busy days!
In the Bahamas we've done it a couple of different ways. We went to Atlantis and did the whole deal with the waterslides and aquariums, etc but we don't think it is worth the $150 that they are now charging for this. We would do a visit to the aquarium again though because that is pretty incredible, if you are thinking of it I would skip the ships tour and grab a water taxi ($6 return I think) and go do the aquarium on your own, then you can do it at your own pace and spend as much time as you like exploring the hotel. The beach nextdoor to the hotel is awesome too and other than the people trying to sell you booze constantly we had a lot of fun there. We also did a ships excursion that had snorkelling and an island tour, it was very good but we would have preferred more time in the water and less time on the bus.