I have one like that. She just turned 9.
She is very hard to shop for/with, as she likes the cargo, chunky looking bottoms, and camo. She practically refuses ANYthing if it is pink or purple, or yellow... any "girlie" color you can just forget it. So we do shop at Old Navy alot. I will get her jeans/pants/shorts there, at Target or at Land's End sometimes. But the tops, I let her do most of the choosing. She usually will lean towards the black, brown or other dark shades (dark red - NO pink tones, dark green - NO neon stuff, etc). She really likes hoodie type shirts though, and since I discovered that (her fav all last year was a grey and black striped one with 3/4 sleeves) I have been able to find some in other colors that look a little more girlie (one is hot pink/red/blue blended threads in the fabric). She also loves the converse or vans skater type shoes. I let her. She's cute. She has super long straight blonde hair, so I figure as long as she doesn't want to cut it short, she can be a little more boyish with her clothes. After all, she doesn't do tea-parties, she is out climbing trees and playing with the dog or keeping up with her older brother. She DID agree to a fabulously feminine hot pink and silver metallic swimsuit this summer though!
I have gotten her to start painting her nails some, so that helps add femininity without compromising her clothing style. She does like the dark greens and blues on her nails though, ;)