A hip, a hip, a hippopotamus
Got on, got on, got on a city bus
And all, and all, and all the people said
You're squishing us, hippopotamus!
I don't remember where I picked up this rhyme, or where the tune I joined to it came from. It has no particular thematic elements, since there aren't really enough hippo - or bus - books around to do more than one themed storytime a year. But you can sing it and sing it and sing it and the kids will giggle every time.
Curve your arms at your side, like a fat hippopotamus, tilting from side to side with the rhythm. Pretend to step up. Curve and tilt again. Wave your arms around, or pretend to be squished and yell the last line.
You'll have to make up your own tune, as I haven't gotten around to recording my voice for posterity yet...