It is not standard and not recommended but a US Citizen can enter Belize via the International Airport without a passport.
Belize Immigration at the airport will issue you a Tourist Permit for $US50.00. You will need valid ID and a birth certifate would help.
The airlines will give you a hard time with this but will let you go. Thus, it is best to call ahead and speak with a Senior Officer at the airport Immigration office, they do not like to be suprized.
S.I.N.