Just got back from a week stay at Ramon's Village on AC. The cave tubing at Jaguar Paw was awsome. We rented a car at the airport (Avis, $55US tax, insurance and everything for a Suzuki Samari with A/C) and drove ourselves. Don't give up on the road. It really is "just a little further", and the road won't swallow your entire vehicle (you'll see what I mean.)
The Blue Hole is a "must do" if you're a diver; be prepared for a twelve-hour day, broken up with a couple of other dives, and a stop at a really cool little island with a booby bird sanctuary.
The Belize Zoo is different from any you've ever seen; just remember to take plenty of mosquito repellant.
Rent a golf cart for a couple of hours, and see the island; don't forget the mosquito repellant as they seem to get nasty on the two ends of AC (no problem on the water, and near town.)
Even if you're not a diver, go snorkeling at Shark/Ray Alley, and jump in with the sharks. It's a real hoot!