Here's one I learned at Cub Scout camp director school. It seems to go over with every age group. It may well be my "greatest hit." The sung part is from the 1929 song, "Singin' in the Rain" (words by Arthur Freed, music by Nacio Herb Brown).

I'm singing in the rain
Just singing in the rain
What a glorious feeling
I'm happy again.

Leader: Thumbs up!
Group:   Thumbs up!

All while swiveling hips and holding up thumbs:
Toody-toddy, toody-toddy, toody-ta-ta
Toody-toddy, toody-toddy, toody-ta-ta

Other motions:
Elbows in
Knees bent
Head up
Tongue out

This is cumulative, so on the last one, you'd sing the chorus, and then hear:
Thumbs up! Thumbs up!
Elbows in Elbows in
Knees bent Knees bent
Head up Head up
Tongue out Tongue out

Toody-toddy, toody-toddy, toody-ta-ta
Toody-toddy, toody-toddy, toody-ta-ta

And, of course, people realize they're rather silly saying this with their tongues out)