I would suggest one of the nicest ways to show your appreciation for a great Cast Member is to write a letter of appreciation addressed to their supervisor. If necessary, just ask your CM who that person would be and the address, since you'd like to send in your positive comments. Then let the pen do the talking!! This could really benefit the CM long-term, with any evaluations they would receive and I'm sure a copy of the letter would be placed in their personnel file.
I experienced a wonderful CM on my recent trip to WDW and wrote a letter to his supervisor, and included my name, address and phone number, and I received a call from a gentleman from WDW stating that they had indeed received the letter, had forwarded it to the CM's supervisor and would be sharing that letter with the CM. It goes a lot further for that person's career than a gift card, although a gift card is a very thoughtful and appropriate gift. Why not do both? Also, if you decide to write that letter, spend the extra few bucks to send it certified mail where someone has to sign for it. That way, they know it's being tracked and you're more likely someone will actually read it and respond back to you if you give them contact information for yourself.