We spent a week with family and friends in January last year at Banana Beach in San Pedro, Ambergris Caye (pronounced "key") to see Jerry Jeff Walker give a couple of shows:
Ambergris Caye Resort – Banana Beach Resort Belize
It was owned by a Canadian family (probably still is).
Rent a golf cart.
Swim with the sharks (not me, thank you).
Go snorkeling. Take a sailing cruise.
Beaches are nice, but there is a lot of marine grass in the shallow water, so swimming off the beach ain't so much fun.
Eat! Drink! Take in some live music. Walk your legs off.
Fly in to Belize City, take a puddle jumper to San Pedro. Make sure you make connections in the same airport (there are two in town).
(Do not stay in Belize City. Rough town.)
There are some good videos on YouTube of San Pedro. It is a beach town. You run out of things to do after a couple of weeks. But a great place to chill out.
It should be plenty warm and humid when you get there.