Haven't been on AC since they were installed, but, reportedly there's an ATM in San Pedro that works with Star and Plus systems - fees will apply. Search the thread to find more information.
We always enter Belize with several hundred in cash - all locations freely accept US dollars, give change in Belizean or mix of BZ & US. I like using travelers checks because they fit in easily with how we travel and what we do. We often stay toward the north end of town and walk along the beach, which makes it dead easy to change checks to cash at Milo's office. The added benefit is that Milo's gives you a portion of the exchange fee as well. Last time we were there we got 205 to 207 BZ for every 100 US. Not a big deal, but, it's better than paying a fee at the bank.... I don't want to sound like a shill, but, Milo's is the best exchange deal we found - on the north end of Front Street, across from a golf cart rental place. (Polo's)
A lot of places accept credit cards - we've used them with no problem. The only catch is that some merchants tack on a transaction fee to cover the 2 or 4% fee charged by the clearing house. We also found that some tour operators were willing to give a discount because we were booking 4 to 6 people together - but they wouldn't do that if paid with plastic.