Have you booked your accommodations for Caye Caulker yet? If not, try this link for a great place to stay www.chateaugiselle.com.
The snorkeling trips departing from Caye Caulker are much cheaper than those departing from San Pedro, for the same stops. The Hol Chan snorkeling trip makes a stop at the Hol Chan Marine Park, Shark / Ray Alley, the Coral Gardens, and San Pedro for lunch - cost $35 U.S. per person. Manatee Watching trip (3 stops: to see the manatees in their natural habitat, snorkeling at Goff's Caye, and a stop on the return at Shark / Ray Village) is $37.50 U.S. per person.
I would also recommend either cave tubing ($107.50 U.S. per person) or the Lamanai Maya Ruins ($107.50 U.S. per person).
You can find a description of each of these tours / snorkeling trips at www.cayecaulker.org/traveltour.
Click on the "tours" link.
Hope this helps.