I think variety is the key to finding something she likes...and being willing to go back to your own childhood to find books.
At that age, my favorite books were "The Black Stallion" series...and the nice thing was that since they were published before I was born, I never had to wait for the next one to come out! Many girls are horse crazy at that age so it may work for her.
Incidentally, the Accelerated Reading (AR) book level for most of the series is fifth grade.
Nancy Drew was another favorite (so was Hardy Boys)...I liked reading mysteries at that age because it was fun to try to figure out the solution before the end of the book. Most of these books are also in the fifth-grade range.
...and what about the Chronicles of Narnia? (Also fifth-sixth grade level).
You might search for ideas at:
http://www.arbookfind.com/default.aspx
You do an advanced search by book level (BL) to find books appropriate for her reading level then look through the list to get ideas for series you may have loved (and forgotten) as a child...and new stuff too.
BTW, if you're not familiar with the system, the "BL" for a book corresponds to the grade and month...for example as BL of 5.7 would be considered appropriate for a student in the seventh month of the fifth grade. I love this site because my son's teacher has given him a goal of reaching the 7.1 reading level by the end of this year...he's at 4.5 now...and he's in second grade!...love this system for developing reading skills...he started first grade at a BL of .4 (fourth month kinder) and hated reading, and AR has helped him just more than four grade levels in 18 months! I found him reading the bible on his own for the first time last week!
Good luck!