A big thank you to US servicemen! Your dedication and service to our home country (no matter how much we love Belize) is much appreciated!
I'll second what Diane has to say. As long as you're only planning a few mainland trips, just get to San Pedro. You can schedule day trips to the mainland from there. Yes, it may end up being a little more expensive per excursion given the travel from SP but there's absolutely no hassle. Most hotels (I think) can arrange the tours for you. Most of the tours will pick you up by boat at your hotel's dock. A couple may require you to fly to BZE. I know the Tikal, Guatemala requires this but don't know of what others.
There are many diveshops to choose from. I haven't heard anything bad about any of them, but then again I only get down once every few years.
As far as which mainland excursions to go on here are my suggestions:
Lamanai Ruins - fun boat ride to Bomba, bus to another river, then boat to the ruins. One of my favorite mainland tours.
I think you can get a package tour of a couple of things in one trip. For example, you can likely go to the zoo and Altun Ha in one trip. Or maybe cave tubing and the zoo. These package tours are available for the items relatively close to Belize City. Lamani is a full day all to itself.
Take an all day Tourneffe or Half-Moon Caye dive excursion. Some of the best dives you'll ever go on, and Half-Moon Caye is beautiful.
A fun snorkel or beginner dive excursion is the Hol Chan Cut with a stop for snorkeling at Shark/Ray Alley.
Just bring US$. Everyone accepts them. They'll give you Belize $ as change. Pay attention to the price you're paying, whether its US or Belize $.