I would use regular money - I'm sure mousekeeping staff depend in large part on tips, and I wouldn't want to limit them as to where they can spend the money. As much as all of us love Disney and Disney dollars, they may just want money to pick up some groceries on the way home.
We also use regular U.S. currency for tipping the maids. We leave it in the bathroom in an envelope labeled for them to make sure that they will take it. If its left out on the dresser, table or nightstand, I've been told that the maids have been instructed not to take it, fearing that its not for them.
I leave regular currency in a decorated envelope that clearly indicates "for our maid/housekeeping" in both english and spanish. The nice thing about using the envelopes - is that I prepare them way in advance when I am sick with waiting for my vacation - and then I fill them with single bills from my wallet each week as the time goes by. Before I know it... I have all the tip money set aside, and didn't even miss the money!
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