Reading Recommendations?

My youngest sister is 9 years old and a voracious reader. She called me today for some recommendations- I gave her a few ideas and she immediately ran out to the library. She ended up with Toys Go Out: Being the Adventures of a Knowledgeable Stingray, a Toughy Little Buffalo, and Someone Called Plastic, Moxy Maxwell Does Not Love Stuart Little, Midnight (Warriors: The New Prophecy, Book 1), and Ivy and Bean and the Ghost That Had to Go (Book 2) (Ivy & Bean). These should appease her for a few days, but I need more ideas!

Admittedly, younger chapter books are not my forte. (I really need to work on that). I am looking for recommendations and your expertise. She reads at a high level, is entering 4th grade, and will give almost anything a shot. Some of her recent favorites include Frindle, the Clementine books, Oggie Cooder #1 (Oggie Cooder), and Lunch Money. Oh, and she read the entire The Spiderwick Chronicles series at the beginning of June. She LOVED them.

I am planning on recommending the Just Grace series next.  But I need more ideas!  I counting on you – do you have any recommendations for great books for 4th graders?