It took us until our third trip to go to the mainland -- AC is so nice. but we went to Altun Ha two years ago, and this time (fourth trip) we took the boat trip to Lamanai (with Daniel, who is a splendid guide).
The Altun Ha trip was fun. The Lamanai trip is a long day and really worth it. We did hear and see howler monkeys, lots of pirds, a baby croc. had a great breakfast of "johnny' cake (journey cakes) sandwiches, good food, plentious drinks, remarkable ruins.
Definitely do it.
A day trip to Belize City is fun, too. We had some stuff to buy for our place, and we took in the museum, which is really fabulous.
Mike and Daphne