Well, I'm a 10 year old girl and I don't even know what I want! I'd say, if she's girly, makeup or nail polish or something called Fijit Friend, check the website. If she's into music, CD's of her favourite singer. If she's tom-boy-ish give her a video game or a soccer ball or a race car. I hope I helped!
Books or music
I found a website that has the best Christmas gifts of 2012 for 9 and 10 year old girls. The website is [ http://www.squidoo.com/best-christmas-gifts-for-9-and-10-year-old-girls ].
A Wii.
$10 - $20
some ting plush
Shirley Temple never recorded "i want a hippopotamus for christmas" - It was recorded by 10 year old Gayla Peevey in 1953.
Trying to come up with original Christmas gift ideas each year can be wearing. Thankfully there are websites out there geared exclusively to Christmas gift ideas for family and those we don't know very well. Since your nephew is 10, try the toy websites for some wonderful Christmas gift ideas.
BooksBoard GamesCandyStuffed AnimalsPetsCard Games
If sufficiently mature, something along the lines of a cosmetics set would be fine.
clothes
maybe get her the new pcd album, Doll Domination
Toy helicopters/toys/trains etc, money, food, something he enjoys.