When it comes to the islands of Belize, Caye Caulker is my favorite. It has a local feel and is a great value compared to some of the more upscale and exclusive neighboring islands. Its easy to get there straight from Belize City via a ferry that leaves throughout the day. It is a destination that suits all budgets from cheap bungalows to exclusive hotels, and its a great place to base out of for fishing and exploring the coral reef. It has a great night life and restaurants. Its a tiny island and the roads are just hard packed white sand. The only vehicles are a few golf carts and beach bikes.
Be sure to check out the bar at the end of the split where you can swim in the current as the tide moves in and out.