We just got back from AC almost 10 days ago and when we go back again we'll bring more cash and/or traveler's cheques! Like others have said - plastic works great for hotels, dive trips, more expensive restaurants and most gift shops...but would bring more cash/cheques next time (or spend less - never!!) for grocery stores, street vendors, less expensive restaurants, tips, etc. Traveler's cheque cashing tip: Western Union on Front St. (Castleberry, Ltd.) worked great for cashing my traveler's cheques (better rate than the local banks). I believe they are open 7 days a week - although hours are limited on Sundays - so check their schedule.
You'll have a wonderful trip!!! Belize it$$$!
Most importantly...the best things in AC are free...the warmth of the sunshine...wonderful people...music and laughter filling the air...still have sand in my shoes and a song in my heart. Can't ya tell?! Enjoy!!