The currency system in Belize is fabulously easy for American visitors.  The Belize dollar is locked at $2 Belize = $1 US.  So it's very easy to see how much something is costing in US when you go shopping.  Most accommodations and tours are listed in US$ prices, and most restaurants, shops, etc. are listed in BZ$. Make sure to verify if prices are in BZE or US though, 80% of them are BZE in touristy places and 20% in US.  Seems like they're trying to make it easier, but it just ends up being confusing.  Foreign exchange at the banks has a lower rate for selling and a higher rate for buying.

Nearly every place (both inland and on Ambergris Caye) readily accepts US currency.  They also all accept traveler's checks as long as you write your passport number or driver's license number on the back.  However, don't bring large bills (anything above a $20) because it could be hard to use $50 bills unless you spend $45 of it.

A lot of hotels, resorts, restaurants and businesses, particularly in the main cities and towns accept credit cards now.  However, in the villages it is handy to have cash on hand for small purchases.