Belize, a little country the size of Massachusetts, is stuck at the southernmost corner of the Yukatan peninsula, surrounded by Mexico and Guatamala, and by the Carribean sea. Despite its small size, it harbors the world's second largest barrier reef, after Australia's. It also sports white sandy beaches, a warm and sunny climate, and some of the richest Mayan temples in the area.
Went down with my sister Maria in January 2007, for an emergency vacation in the middle of a dozen projects before the spring term starts. Climate-wise, the dry season doesn't start till February, but we got lucky and the weather cooperated (although it had been raining for 5 days before we landed).
You land in Belize international airport on the mainland, take a little shuttle airplane (leaves every hour for $45) to wherever your destination is (Caye Caulker, San Pedro, southern cities), and off you are in your beautiful vacation (don't bother with the water taxis).
Caye Caulker is a backpacker's island for the light traveler, San Pedro (Ambergris Caye), is its northern neighbor for $100+ a night. Diving, water sports, daily excursions filled most of our days. Then day trips to Lamanai (avoid Tanisha Eco Tours), the Blue Hole (choose Amigos Del Mar), or the coral reef (lots of amazing sites 5 mins away). Highly recommended!
The Blue Hole is one of the most famous dive sites on the planet. It's a rim of sand in the middle of a coral sea, going down diagonally (pretty steep) until 45 feet, where the sand drops vertically down to 500 feet. At about 135, an opening gives way to stalagmites and incredible underwater formations which once formed the ceiling of an enormous (cenote) cave.