When I remember, I like to add a joke to my first grader's lunch box. He loves telling it to his friends (as if they need an ice breaker for the dinner hour!)
Usually I just google "riddles" or "simple jokes" the night before and find one that's age appropriate.
Q: I am lighter than a feather, yet no man can hold me for very long. What am I?
Q: Feed me and I live, give me something to drink and I'll die. What am I?
One of my favorites from the past:
Q: What does a termite eat for breakfast?