This website http://wiki.kcls.org/tellmeastory/index.php/Category:Seasons has so many great rhymes, not only for Autumn, but all seasons! Below is one of my favorites.
To the tune of Frere Jacque
Fall is here. Fall is here.
Yes, it is. Yes, it is.
All the leaves are falling. (wave hands)
All the leaves are falling.
Crunch, crunch, crunch. (stomp)
Crunch, crunch, crunch.
for a different variotion on this rhyme, gather up real leaves outside and let your child actually jump and throw leaves!