Here's a short list of the things my cats find amusing:
Destroying a loaf of bread accidently left on the kitchen counter. Method: Bite through plastic, poke numerous holes in plastic with claws, gnaw through plastic, shred bread slices with claws. Walk off with a bored innocent look.
Tearing into an unopened bag of townhouse crackers, crunching the crackers into bits, but not swallowing any it. Lie in pile of crumbs until they stick to fur, then roll around on the sofa.
Shredding full paper towel rolls. Method: Grab roll with front legs, lie on side, rabbit kick the roll with back legs furiously until small bits of paper fly in the air like a snow storm.
Force leg under any closed door, cup paw and pull like mad in a fake attempt to open the door. *They know the pounding noise will make a human appear* Must be a cat magic trick.
Wait for me to brush my teeth, then jump into the sink the second I turn the water on, so they can splash. Leaving me with a mouth full of toothpaste and no where to go with it.