When my daughter was much younger, she was a very serious child. She wasn't really into the laughing/giggling thing, she rather preferred to frown or cry. This drove me nuts. I'd go to herculeanly-silly neck-wrenchingingly-slapsticky efforts to eke a chuckle out her. Sometimes it worked.

When she got a little older, I quickly introduced bathroom humor. It's much easier than flinging yourself around the room and it's a classic. Typically this would take the form of me inserting the word "poop" into inappropriate places, like song lyrics:

"Twinkle twinkle little POOP!"
or
"She'll be poopin' round the mountain when she poops!"

This didn't always get a laugh, but it would usually result in a satisfyingly (to me) silly conversation.

Those days are finally over though. Now, when I bust out a "The itsy bitsy POOPY went up the water POOP", she just looks at me sternly and says, "That's potty talk. When you talk like that it means you have to poop or pee. Go poop, Daddy."

This smells like school learnin'. Boo for school.