Belize doesn't really have all that much in the way of nightlife. The capital city is small and relatively quiet by most international standards. In both the capital and throughout the rest of the country, you'd be hard-pressed to find a truly notable bar or club. Still, some unique after-dark destinations and activities are out there, and should not be missed.
Stargazing: This is one of my favorite nighttime activities, but it is especially rewarding when there is no (or little) ambient light. Given its sparse development and low population density, Belize offers a wealth of opportunities for some truly spectacular stargazing. Your best spots are the isolated beach getaways of Belize's three mid-ocean atolls, but you can also enjoy the astronomical splendor from any number of deserted beaches or rural mountain getaways.
Night Diving: If you've come to Belize to scuba dive, you should definitely try a night dive. Many creatures are nocturnal, and the reefs here come alive at night. Moreover, the brilliant colors of the coral and sea life really shine under the strong glare of an underwater light, and there's something truly eerie about the experience. All of the major dive destinations and resorts offer night diving.
Barefoot Iguana (Ambergris Caye): This cavernous bar features everything from hot new DJs to mud wrestling to sporting events shown on a giant screen. The two-story hangar-sized space is hung with faux jungle plants and foliage, and it has quickly become the most happening nightspot in San Pedro.
Sugar Reef Lounge (Placencia; tel. 523-3289): Placencia is a small village. On Wednesday nights, it seems like the entire town heads to the Sugar Reef Lounge for their karaoke night. On other nights of the week, this is also one of the better spots in town to hang out after the sun goes down.
Moon Rise at Tikal: Watching the full moon rise from the top of Temple IV in Tikal is one of the highlights of my many travels to this region. You'll have to stay at one of the hotels on-site to do this, and you may even have to persuade or bribe a park guard. You'll also have to time your visit with the moon phase. But if all these things come together, you're in for a memorable and awe-inspiring evening.