It's been a few years since I've been to Belize but here is what I remember. Most of this is non-diving related.
o Liveaboards in general - don't pack too many clothes! It's pretty darn casual and you'll be in your swimming suit most of the time.
o If you are person that gets cold easily pack a sweat suit or some type of jacket. The inside portions of the boat will be air conditioned and I find myself getting pretty cold.
o If you are a photographer make sure to take some slide film. Many of the boats have slide developing and you can get pretty fast feedback / help with your photography.
o If this is also your first warm water trip - remember to pack the essentials in your carry-on. Many of the locations do not have daily flights so if your luggage gets misplaced you may be without your stuff for a few days. (Been there and sure was glad I had my regs, bathing suit, shorts, computers and the like.)
o I didn't particularly like the Blue Hole. It just seemed like an excuse to go a bit deeper. There were some rock formations but seeing you live in the mid-West I'd say Bonne Terre is a better use of your time.
o There was a small island (caye) with a lighthouse and bird sanctuary that made a nice surface interval.
o If you have time beyond the live aboard there is a day trip to some ruins about an hour away. Can't remember the name (it has been a few years!) but it is the ruins that are on the label of the local 'Belizian' beer.
o If you are extending your trip a few days you could venture to Chitzenitza. I did these ruins when in Playa Del Carmen a couple years ago. The best explanation was given by a tour guide.... If all the other ruin sites are considered a city, this is NY city. Very impressive.
o Last year when on a cruise Belize was one of our stops. I went on the best excursion on the trip there. It was a jungle walk and cave tubing. We were each given an inner tube and a head lamp. Hiked for about an hour through the jungle, swung into a river via a rope, and floated down the river through some really cool caves.
Hope that helps. & have a safe and fun trip!
Paula