I read your great trip report and got some fantastic ideas. In addition, since I only have a day and a half, after diving for a week, to do one or two really special things that's what I need help with. The Nekton will take us to the ruins or the zoo. Is it worth the trip? We were thinking of cave tubing, but wanted a half day trip instead of the full day since we're all going to be relatively tired. Nobody offers a half day.

Logistics are going to be a real problem for you with the limited amount of time. Most of the places to see on the mainland are spaced far enough apart that travel time is pretty lengthy. Remember that a good road in Belize is not quite as good as a bad road in the States.
The cave trip might just be too far away to do in such a short amount of time.
Enjoy yourself as much as you can, but don't try to do too much; it just won't work in the tropical sun :-)