The buses leave Belize City every 30 minutes, take the Benque/Belmopan bus, it will cost you about $2.50 US to travel the 1 hour trip to the zoo. So no need to overnite. The Belize Zoo is a great Zoo, I am not much of a Zoo person. You can get off the bus and walk into the Zoo. Then walk back to the road and wait for the return bus. Dont do this on a Sunday as the buses dont seem to run as often. The Zoo has a big gift shop, ice cream ,food and drinks etc. Have a great time.