We were in Belize in August 2004. I agree leave the kids at the kids club, and then go to Belize. We did the excursion that was Altun Ha and the river boat ride back. My husband and 2 girls loved Altun Ha. (Personally I thought it was a pile of rocks) but they all thought it was fabulous. We took a native river boat back. Saw lots of monkeys, a few crocs and some manatees.
The bus ride from dock to dock was good too. Air conditioning and lots of local beer. Belize is very poor and dirty. My then 10 year old was very upset by the poverty. It's worth it for an adult to visit, but I recommend leaving the kids until they get older.