You can certainly chat with folks before you go down, maybe see what days they are going to Lamanai, or manatee watching, some tours don't run every day. Peek at: //AmbergrisCaye.com/tours/
and go to some of the folks' websites who do the tours and become familiar with whats there, say if you wanted to do the kayaking the back side thing with Seaduced by Belize, its awesome!
or Lamanai with Daniel: http://TanishaTours.com/lamanai.html
SEArious Adventures also do the Lamanai trip: //AmbergrisCaye.com/searious/
as does SeaDuced by Belize as mentioned above for kayaking.