The folks at Clarissa Falls are great, but prices have really gone up there. Depending on which lodging you get, it could cost you almost as much as staying at one of the better jungle lodges. Ian Anderson's is great for active travelers and does do one-day as well as longer trips.
I think most people would agree that Actun Tunichil Muknal is the must-do accessible caving experience. Lots of other things of interest to kids as well -- canoeing, waterfalls in the Mountain Pine Ridge, Maya ruins at Caracol, Xutanunich and elsewhere, Green Hills butterfly farm and of course back at Mile 30 of the Western Highway the Belize Zoo.
--Lan Sluder