You definetly want to have some cash on you and smaller bills would be better. Upstairs in the checkout building there is an ATM and they will break your large bills, if that is what you bring. You can tip the spa people and people who help with your luggage also.
I'm not sure about the other resorts but CSA is cashless. Meaning they will swipe your credit card for $300 for purchases and phone calls. It's a hold like every hotel does. At the end of your stay, you will obvisouly only be charged for your purcahses. When you need something from a store, they will jsut ask for your name and room number and you sign for it. Presto! It's that easy! The cashless system is nice!
We took the shopping trip and many of those places took credit cards. Maybe it's me, but I would prefer to use cash in a foreign country.
Resort - You charge everything to your room and at check out you can settle with your credit card or cash. (USD)
Off resort - Use only cash in small bills. Take ones, fives and tens. Anything bigger they may not have USD to give back to you.
Pixel, Couples needs a credit card or debit card with a Visa, MasterCard, American Express, Discover logo on it. I do not think they take checks. A US or a British check would take months to clear and have a lot a fees. It also would be very hard for them to collect if it bounced.