A couple of years ago I was given this cute thank you gift. It would be simple to make. Just purchase a men's tie box (from the mall) and line it with tissue paper (or similar box). She used vanilla wafer cookies and painted the tops with white chocolate. Once it dried, she painted the piano keys with melted milk chocolate. It was so cute and clever. You could attach a bow and small card on the front thanking them for their "sweet" service.
You could make this gift as a handout or as a thank you for your Primary/RS pianist, piano students, etc.
UPDATE: I made these recently as a gift and used the suggestion in the comments below and used snack size Kit Kat bars. I cut them in half and made them look like a keyboard. I didn't even dip the Kit Kats in dark chocolate (as was suggested in the comment section below). I just put them in place right after I put the white chocolate on...that way once it hardened they would stay in place. I lined each box with wax paper and then folded it over and put the lid on.