I would suggest Capricorn for a romantic dinner. Beautiful area, pleasant boat ride to Capricorn and very good food.
Or Rendezvous. The food and wine are excellent. Also a short boat ride.
Another fun thing to do is take the water taxi to Caye Caulker and spend an afternoon. Walk around town, walk down to the split, have a drink at the Lazy Lizard and enjoy the amazing view.