The great thing about the kids I sit for is that I can get them totally stoked and ritualized about anything. My latest is belly buttons, which I grew up calling "bling-bling." So I've got them all wired: "Where's the bling-bling?!?!?" And they gleefully pull up their shirts to point it out.
I decided that wasn't entertaining enough. I noticed the the girl, always the overachiever, lifts her shirt up as far as she can get it, so I've conditioned her to raise her shirt when I call out, "MARDI GRAS!"