Hop like bunnies
Run like cheetahs (really fast)
Crawl and bark/meow like dogs/cats
Waddle like penguins
Fly like birds
Swim like fish
Walk like an elephant (use your arm to create a long nose)
Crawl and oink like a pig
The only limits in this game are what you can come up with. I have no idea what sound an armadillo makes, so I'm not going to suggest it. As the kids get older, you can have them swim like whales, but also make whale sounds (think finding Nemo) or just watch the following video.
This is sure to bring lots of giggles. Have you tried anything like this? What's you favorite animal to imitate?