KITTENS!
A pair of hyperactive playing kittens... they will pounce on ANYTHING at this age just for the hell of it to see if it will jump. So chaotic and hilarious while tripping, so BEWARE! Throw in a laser pointer, or ball of yarn, or something and you may just give yourself a hernia from laughing so hard.
Puppies too of course. Even better, Kittens + Puppies.