I have never really been a destination diver, but more of an opportunistic one: When there's a dive shop and good diving and I have time, I'll try and fit in a dive or two. The Blue Hole in Belize is an exception, however. Two hours boat ride from the nearest caye through open water, the Blue Hole has to be something you are determined to get to. And it's worth it.
It's a deep dive (120 ft/40m) so quite short (around 8 mins at the bottom), with not so many fish or coral, although sharks are common. What I loved the most were the massive stalactites that we swam through at our deepest depth -- bigger than I've seen in most land caves, but very unique to be seen underwater (they only grow on land, of course). It was formed as a cave in the limestone millions of years ago and collapsed into a cenote. With the rise in sea levels most likely just after the last ice age, it is now underwater in the middle of a pristine, magical reef (Lighthouse Atoll) near the second largest barrier reef in the world (running the entire length of Belize's coast). Amazing place!
Backbacker heaven: a strip of sand in the middle of a calm, tropical ocean, with reggae music, cheap beers and lobsters, simple living, swimming, diving, snorkelling, windsurfing, kitesurfing, sailing, and a friendly nightlife. This is Caye Caulker, a small coral islet out in the Caribbean so small (5km x 1km) that you could circle it in a couple of hours, and if a big wave came, it would obliterate everything. Hurricane Hattie (1961) did almost just that, but the island reemerged, albeit cut into two islands with the split just north of town (the split that was created was only a few feet deep, but this was deepened by manual dredging and subsequent tidal flows). Hurricane Keith (2000), also caused considerable damage. Here, the most challenging thing might be navigating the protected sea grass that grows along the shore or deciding to dive off the docks instead. 
The main jumping off point for the famous Blue Hole (#956), and for those not diving trained, the closer Shark and Ray Alley (#930), it's also a mid-way point for travelers on the way to Ambergris, a more developed cay further north. It has two main north-south streets -- Front Street and Back Street, which are only a few houses apart.
Caye Caulker was first settled hundreds of years ago, but mostly dates from the Caste War of the Yucatan in the mid 1800s when mestizos fleeing violence moved to the island to settle. It first emerged as part of the Gringo Trail in the 1970s as tourists moved from the Mexican Yucantan hubs to Guatemala's Tikal, and the easily available marijuana certainly helped its popularity.
Surprisingly, it's only 20km from Belize City, has it's own small airstrip, but the main form of transport on the island is, of course, walking, although the odd cyclist can be found.
