RE: Credit cards
You have to work pretty hard to find a place in AC that won't accept your credit card. (Perhaps the fresh orange juice and bbq chicken guy on Back Street won't). I've never seen any type of conversion charge on my cards, but that's not to say it doesn't happen. We've never returned with more BZ$ than we wanted to bring home. A few days before you are scheduled to return start asking for your change in US$ and most of the local businesses will accomodate you.
If you are departing, the Jet Bar at the airport will ALWAYS give you change in US$.
Of course if you keep a lot of BZ$ it is a great excuse to go back..