You can fly into Belize City Municipal airport (not the big one you fly into from the US) and back to san Pedro for about $50 each. then get Ian Anderson's Caves Branch or Jaguar Paw to pick you up, or take a bus out to their place. Ian Anderson's has a shuttle on several days of the week.
They can set you up with all kinds of hikes. I love it there. the grounds are incredible. great river with rope swing, we did the black Hole drop one day, whew, that was exciting. 280 feet off a cliff on a rope. none of us had ever done anything down a rope. my wife and i, and our two teenagers and two friends of thiers. i would have bet the ranch my wife would NOT go off that cliff, but over she went.
the jungle is incredible. the two hour hike thru the jungle to get to the cliff was gorgeous. Going to Lamanai you get to see some jungle-ish stuff, but not like the hikes we went on at Caves Branch. http://AmbergrisCaye.com/pages/town/cavesbranch.html