You can get baht from most ATMs at a decent rate, but beware that your financial institution may charge an additional conversion fee, and the VISA or MasterCard affiliated checkcards and credit cards also charge a conversion fee.

Since your Phuket stay is rather brief, rather than the long ferry ride back and forth to Phi Phi, I would recommend a longtail boat ride through the mangrove waterlands and out to James Bond Island. I think it takes about half a day and provides plenty of photo ops.