Adding to the above suggestions......
The Oasi (
http://www.holidaybelize.com/) is a great place, quiet and has bikes, etc. you can use.
Iguana Reef Inn (
http://iguanareefinn.com/index.htm)is in my opinion the nicest place on the island-managed by real pros.
You never said what your budget was, but there are getting to be some really nice-what I would call upper end places on the island.
cayecaulkerbelize.net
gocayecaulker.com
Both of these sites have a map with links to most of the motels on the island.
Mark