Quote:
I don't want her to just sit around and read all day.

Honestly, I think that's one of the
best things a kid can do with their extra time!
But I also like the research project idea, or even encouraging her to write a book!
Ask if she has any idea what she wants to do when she grows up.  If she wants to be a teacher, for instance, maybe she could help out in a younger grade (with her parents' permission, of course!) If she wants to go into science, or history, or whatever, that would help narrow down research topics.
Maybe as enrichment in math, she could write word problems to go with what your class is studying? Exercises her math and writing skills, and kids always like problems better if they see their name and things/places they know in them.
Some of my early finishers over the years have enjoyed memorizing things as well. I had a 3rd grader get to about 30 digits of Pi one time! (There's also the periodic table, states and capitals, countries and capitals...)