We do that - we have business-card sized cards that we print at home (you can get the blank cards from an office supply company or
Amazon) and then we put a little lapel pin of our state flag on there. The card says "You made our visit more Magical!" and our family name, and the dates of the trip. It's something tangible that we can give to CMs who otherwise can't be tipped. (Full transparency: we give them to *any* CM who we feel gives us exceptional service or makes a Magical moment, or goes above and beyond - from bus drivers to wait staff, to Bell Services.) We love to give them out (the CMs love getting little "tokens" like that) and we try to give out at least 10 every trip, and it's usually not hard to do!
We also make a point to stop at Guest Relations if we have an excellent experience, and let them know as well, so that the CM gets real, proper "credit" for their awesomeness! And they love to hear those stories - since they spend so much of their day with folks who are grumpy, I'm sure it's nice to work with someone who is there for a happy reason!