The Zoo and Xunantunich in one trip is a lot.
The way I always recommend the zoo to people, though, is go on the last night of your trip. Sleep there in their Tropical Education Center and go on their amazing nocturnal tour (the cats and a lot of other animals are much more active at night and a small group will likely have the place to itself). They serve an excellent dinner and breakfast and they will take you to the airport in the morning. It's a great way to end your Belize trip