unfortunately San Pedro does not have full gyms with a wide variety of weights, cardio, and classes.
Oscars has the most weights, yet rusty (so wear gloves or else rust on your hands), some equipment has duck tape on it, dirty, does not smell great.
San Pedro Fitness Center is above Belize Yacht Club. A/C, clean, yet less weights but enough for a weeks workout and for cardio a treadmill, stairmaster, and precor-type machine, and 2 bikes. Use of the tennis courts and pool too.
It is the only thing I can think of that this wonderful place is lacking, one day they will have a nicer gym. Although if this is just a short vacation you'll be fine and enjoy walking or running the beach I'm sure.