Mousekeeping envelopes are envelopes you leave your mousekeeper's tip in each day. My DW and I use plain white envelopes. We downloaded some Disney clip art and then printed it on the envelopes using Microsoft Word.
We print a message on the envelope -- like "For the Best Mousekeeper at WDW" or something like that. Inside we sometimes put a little note saying "thanks for taking care of our room".
Several times our mousekeeper at WL left us extra towels, soap, shampoo. On a couple of days, the mousekeeper made towel animals for us. I left my Tigger toys out one day and when we came back, they were all wrapped in hand towels. The mousekeeper left a note card. Inside it said: "Tigger says thanks for bringing us to WDW. We had a great time today at the spa!" It was terrific! Of course, they were laid out on the bed, the TV remote nearby, and the Disney channel was on!
There are a number of clip art sites that feature Disney characters. Just do a search on "Disney clip art".