Glover's Reef is one of the finest reef systems in the Caribbean — a complete atoll about 45 km offshore from Hopkins, with a vast shallow lagoon ringed by coral walls that drop hundreds of metres into the deep blue. It is part of the Belize Barrier Reef World Heritage Site, and the diving here is outstanding: wall dives, healthy coral in the lagoon, and open-ocean conditions on the windward side that attract larger pelagic fish. Snorkelers who don't dive can work the shallow patch reefs inside the atoll, which are thick with sea turtles and rays.
Day trips from Hopkins are possible — the crossing takes about an hour depending on sea conditions, and several operators in the village run them. If you have more time, multi-day camping trips to the atoll's small cayes give a very different experience: sleeping under stars, snorkeling on your own schedule, almost no other visitors. The atoll is formally protected as a Marine Reserve and Managed Resource Area; entry fees apply. Plan around the weather, as the boat crossing becomes rough in strong easterlies.