If you want to operate at WDW on a cash basis, instead of using your credit card, you can put cash down to charge against when you check in, just like you would a credit card. I've done this several times (I've also done the same thing with traveler's checks).
This has a few advantages: it's a lot more convenient charging things to your room key than hauling a lot of cash around, you get an itemized list of what you spent your money on, and no worries about the cash getting lost or stolen.
If you don't spend all the money you put down, they'll give you the rest back in cash when you check out (even if it was traveler's checks you put down).