If she's still into vampires, I know some of my 5th graders after Twilight began to read the "Marked" series. It's a tween read and not childish. I have not read it personally, but I've heard good things about the series.
I personally have enjoyed the Peter Pan series, starting with "Peter and the Starcatchers".
Check out my class's Donors Choose page: Wildcat Projects **Donors Choose is a place where teachers post projects for their classes and request materials and/or technology for that project. People then can make donations starting at just $1 to help fund that project. This is a great way to support teachers and their classes!**
I did like the Seekers series (the one with the bears, go figure).
I'm also big on the Tales of the Otari which is a Chinese/Japanese hybrid. Those were really tastey.
Can you tell I teach language to tweens?
Narnia is a classic, but the heavy symbolism upsets some parents. Roald Dahl is a fantastic author and there's a ton of activities on the internet to follow up. I taught Charlie and the Chocolate Factory and then we tried different chocolates from around the world---Cadburys were a favorite--to emphasize each chapter.
My kids had no idea what a gobstopper was. Or a jawbreaker, for that matter.
THe Phantom Tollbooth is also a fun one to work with. Each chapter is almost like a complete tale, so it works, just like Harry Potter.
Gary Paulsen is a great, masculine author and is very famous for his book Hatchet, about surviving alone in the wilderness. However, I liked his books about dog sledding better, like Dogsong.
Holes is very good; the movie is just as strong for follow up.
Garsh, that's all I can think of now. Do you have an accurate reading level? And does your local library have a summer reading program? Sometimes those have great supplementary activities that keep the kids interested. Another idea? I try to pick books that have an upcoming movie...say, like, A Christmas Carol... and then read the book with the youngsters and then see the movie to discuss the differences.
Has she done Lemmony Snicket's Series of Unfortunate Events?
Jerry Spinelli books (Maniac Magee)
Black Beauty (if she is into animals/horses)
Louis Sachar books
Last of the Really Great Whangdoodles (by Julie Andrews)