Lynette,
How thoughtful of you! My daughters always like to take thank-you's as well. Disney Cast Members go above and beyond so often, it's really nice to have a way to recognize people who have helped to make your trip a little more magical.
In the past I have printed out cast member appreciation cards from
www.themouseforless.com. On the left side you will click 'downloads', then scroll down to find the printable you want. Be forewarned, they are addicting! I was just there last night printing out Disney word finds, trivia questions and answers, Epcot educational games for the kids, and much more. (I have you beat - we leave in 6 days!) You do not have to register to be able to print these out.
www.disneyworldtrivia.com also has a downloads section with cast member appreciation cards, but you must register for their website in order to use them. It's not a bad idea to do anyway - it's a great site!
My favorite cast cards, though, are found at
www.tagrel.com. They have a nice variety of characters, sayings, and occasions. You can print cards specific to Halloween or Christmas, for example.
Have a wonderful vacation!