There is no need to exchange at the banks. The merchants will gladly accept USD and will give change in BZ. If you play it right you will not be left with an excess of Belize money upon your return to the states.
Me too AZ, I am looking at $30 BZ right now that I have to spend at Jets bar when I get in! And as always... I will return to the US with a $20 BZ as well as a $1 BZ as a lucky tailsman to get me back to SP!