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.
Both of these sites have a map with links to most of the motels on the island.