Luggage and bags:
Pack light and don't let your bags out of your sight.
Clothing/Shoes/Weather Gear: It is always warm; the spring and early summer is always muggy (not to mention being hurricane season).
Toiletries and Medical Supplies: You should watch the old Harrison Ford movie 'The Mosquito Coast' before visiting. It was filmed in Belize. Bring lots of sunscreen and insect repellent. There are occasional cases of malaria in Belize. AIDS is a significant problem in this country.
Photo Equipment: I prefer bringing the disposable cameras when traveling to Belize. If you beu the waterproof ones, you can use them while snorkeling or swimming.
Camping/Beach/Outdoor Gear: There is no decent beach in Belize City per se. Most people catch a motor boat to Caye Calker for the chance to swim or snorkel.