Most Beverly Cleary books are actually somewhat harder, like 4th/5th grade level. They're fabulous for advanced readers, though, because the content is pretty young and innocent.

My DS5 really loves the Magic Schoolbus chapter books. They aren't great literature, but they're better than Magic Treehouse, IMO (those make me want to end it all if I have to read one aloud). I think they're 2nd/3rd grade level.