Captain Underpants - yes there is some misspelling in the comic portions, however they are really strong on imagination and problem solving!
Frannie K. Stein, Little Girl Mad Scientist - My boy loves these, she is not too girly, it's mostly icky science fun.
Calvin and Hobbes collections- Fantastic level of vocab., for kids who love to figure out big words. Can be put down and picked up anytime. My son practically wears his Calvin and Hobbes books. And they are of course hilarious even for adults.
For more advanced reading level, but still a large serving of silliness: the Wimpy Kid books. There are only 2 in the series so far, but more are planned.
