Gongrats Mark & Marci!
FWIW...
We were in San Pedro City (Ambergris Caye - Belize, C.A.), just this past April. If you PM me I'll provide more detail, but here is what I will offer you in this reply...
[1] We stayed at the SunBreeze Hotel and dove with Aqua Dives. Don't know (by experience) about some of the others, but I will say that Aqua Dives has it all. The best boats, crew, DMs, the most boats, dives, etc. Also the among the best prices.
[2] Hol Chan (hole in the reef that makes a channel to the open water) reserve is a great night dive (shallow)!
[3] You really should do the all day Blue Hole / Lighthouse reef dives. Ask to go to the Half Moon Caye Wall and "Aquarium"
[4] All the dive sites are interesting around the San Pedro reef area.
[5] Try the local eateries next to the Church in the middle of town. Excellect food - bargin prices - the best smoothies from the vendor on the end (looks sleazy and delapidated, but don't let the looks fool ya!). Makes a great place to get a cheap lunch so you can afford a nicer place for dinner!
[6] Food at the Blue Water grill is outstanding (somwhat pricey - but not out of the ordinary for most of the local resturants which have a nicer atmosphere - located at the Sun Breeze Hotel on the beach)
[7] Buy some of the local fruit fromthe street vendors and take along with you on the dives - yum-yum-yummy!
[8] There's a pizza joint with great pizza at a great price out by the aeroport (forgot the name - but easy to find)
[9] Do a Sunday service at the church in San Pedro (if you're there on a Sunday)
[10] There is little to no equipment sales/service for anything dive related. If you bring your own, make sure you come prepared with silicone grease, any special batteries, spare parts, etc. Especially with respect to camera/video gear.
[11] If time affords, the Mayan ruins are cool, but the ride out and back is long and torturous - I wouldn't bother if time is short, you value the dive time too much, or if you don't have much interest in that sort of thing (my wife did - yesh!)
[12] At least at the season we were in, the waves are quite high - beware that you may face some challenging entries and if you stay at the surface too long and are any bit prone to sea sickness - be prepared!
[13] The diving there is so beautiful. If you do have UW video and/or cameras - take em! Use em! If not, don't misss the chance to capture some great memories by visiting one of the local shops which can accompany you on your trip and capture some video for you. It's worth it!