LOVE Captain Underpants and Super Diaper baby and Dogzilla and Cat Kong and Dav Pilkey's books in general. Harold and George get into such delightful mischief.
There was a page of flip-o-rama instructions in the first Super Diaper Baby book that said, "You know, since nobody reads these pages, we figured they'd be a good place to insert subliminal messages" It then urges kids to think for themselves, boycott standardized testing, read banned books and so on. Awesome!
Although my kids are older and have moved beyond these as far as reading level goes, they still enjoy them for their humour. I recently got my seven year old nephew hooked on them too.