Here are the things we've done and truly enjoyed.
Drive cart north and south as far as you can go without undue damage or geting stuck, great way to see lots of fun things.
Talk to locals and ex-pats to get advice on restaurants, TONS of wonderful places to eat and many very reasonably priced.
SCUBA, snorkel, walk the beach, walk thru "downtown" and absorb the wonderful village atmosphere.
Caye Caulker and ride the boat and day trip with the local guy that hangs out at the Lazy Lizard named Razz Creek.
Fishing either on the reef or fly fishing (haven't done the FF yet but hope to this coming week!)
On the mainland you have cave tubing, zip lines in the jungle, spa treatments at Maruba, rides up the rivers on cool fast boats, the zoo, climb and tours the Lamanai ruins.
I could go on but you get the idea. Come here and have fun!
Some of these folks didn't get enough fiber today, they're usually very nice. Be sure to come back now, ya hear!