I also put the majority of my purchases on my room key. That way, everything is charged to my credit card as one lump sum instead of a bunch of smaller purchases. Also, I earn the rewards points for using my CC. I only carry a small amount of cash to pay for things that I can't put on my room key (water, ice cream, etc.). The most important thing to remember is, though, is to have the money already in the bank. That way, when your CC bill comes, the money is already in the bank to pay off the bill. Do not charge anything against your room that you can't pay off once your credit card bill comes.
Hope this helps!