A couple of things:
One, if you're diving with Nekton, you don't get anywhere near San Pedro. You won't be able to drop anything off for the children from where you'll be, unless you dash to the airport and hop a flight out to the island--which isn't a bad idea, really. You'd have time to spend the day there. Go out from Belize Municipal, and fly back to the International airport--it'll be quicker; unless you have to go to BZE to check your bags first.
Two, it took them forever to get all of us off the boat and to the airport--three trips in their little bus, each one taking more than an hour. They do offer a city tour with a guy named Pete, I believe. A few of our group went with him and found the tour quick but mildly interesting.
For those who had later flights, Nekton is now dropping you back at the Princess Hotel and Casino which, if you've been there, has some amenities available. Lunch at their outdoor restaurant was pretty good, the casino is pretty lame and much of the hotel is undergoing renovation, so I'm not sure what will actually be open. They have a three-screen movie theater with semi-current releases, but only one of them was functioning a couple of weeks ago.
Unfortunately, your time slot doesn't allow for much past that.
immissing's suggestion of looking at the SPSC's project is a good idea. They're looking to help the kids on AC in some much-needed ways.
* I Go Pogo *