We just spent just over a week in Belize this last February. We split our time between San Pedro and San Ingico (in the Mayan Ruin) area on the Western highway. San Pedro was great. You will have to take a boat to do any diving, but two tank dives, including the tour, dive master and equipment (BC and skins) were only about $40. Snorking also required going out on a tour boat to get to the reef areas, but again it was not very expensive. We only went to Caye Caulker once for a few hours. It is much less developed and more laid back. That is not to say that San Pedro is not laid back, it is just a bit more developed. In San Pedro, we rented a golf cart (it is the transportation mode of choice as I would guess that almost all rent golf carts), which allowed us to get aroundf the island easily.
We also really enjoyed our 4 days in San Ingico. Our family enjoyed the Mayan Ruin sites much more than we anticipated. They really are truley amazing. We also did the zip line and cave tubing tours that are in that area, and a spent a lot of time hiking and enjoying the wilderness areas.
All members of our family, my wife and I, along with two adult kids (18 and 21) consider this Belize vacation to be the top vacation our family has ever taken. The two areas were truely diverse and gave the feel of two differing vacations (one water based and the other wilderness based).