Take the water taxi to caye caulker for a day. Hang out at the split and Lazy Lizard. Water taxi is $35 US for the two of you.
Take the water taxi from Belize City also, it will save you some $$.
Instead of golf carts rent bikes for a couple of days and explore the island. I like them better than carts. They are better for my back.
Take at least one snorkle trip to Hol Chan. It will cost $50-60.
I liked TQs advise, just added a couple of things.