Head, Shoulders, Knees, & Toes... Knees & Toes
Sing the song.
Then sing it faster. Then faster again. Then lightning speed.
Then slow. Then slower again.
Sing it loud. Whisper the song.
Sing some words loud while others are soft.
Sing boys on some words and girls on other words.
Mix it up. The more mixing you do, the more the kids will enjoy it.
If you don't know the song, here is a rough version.