I have a six year old girl - and I second the notebook idea! Get the small ones that are in the shape of a flower or heart or have sparkles on the front, etc. Kind of diary-like without the lock. And a cute or fun pen - my dd likes the ones with fluff or feathers on the end....
If dd gets one more Polly Pocket I will lose it. She has so many of them and they are hard to dress even for an adult!
Paperback books are a good inexpensive idea but hard to know what they may already have.
Art supplies are fun - glitter glue, fun scissors that cut a pattern or a book of art projects. My dd was in a Scholastic club for Hello Kitty art projects and that had great, easy, fun ideas and came with the supplies to do it. Wonder if they have those books in the regular book stores?
My dd loves nail polish but not all moms are happy to see that come in the door. Hair accessories are always popular with my dd - headbands, ponies, barrettes.