All US citizens are required to present a valid passport for travel into Belize. No vaccinations are required.
The weather should be sunny and warm. You'll need sunscreen during the day, and I'd suggest having layers at night.
YES.
While the Belizean dollar is used, they do accept the USD and all major credit cards.
Yes, tell your bank you will be traveling to Belize so that you can use your cards without getting them cancelled.
If your invitation says “and Guest,” then yes, if not, we would prefer if it was just you.