Okay, I have a just turned (in Sept) 7 year old son, so will try to help with some "generic" gift ideas. At this age they are really into reading "chapter books" (vs the 15 page storybooks). One series that is really popular with my sons class is the "Junie B. Jones" books. (I even get a kick out of these) Any Borders or Walden book store will have the whole series. Another good one is "The Magic Treehouse" series. These will come in handy for the long car trip too. My son and I drive 450 miles to his Dad's state for Thanksgiving and Christmas, so anything that can be used for entertainment in the car is a great gift. Kids this age are really into arts and crafts, too. Maybe make up a portable art kit that she can use in the car. Most of the "Dollar Stores" have these nice little stencil and sticker books, washable markers, colored pencils - LOTS of paper. One thing my son is asking for to use in the car is a "Video Now" - like a small portable DVD player that only plays special Nickelodeon discs, but I haven't checked that out yet, so don't know the cost.
LOL - just checked with my son on what a girl would like - he says get her some of that "stupid makeup, dressup stuff" - so, I don't know how you feel about that, but I know the little girls love nail polish and all that "make me pretty" stuff.
The GameBoy is a great idea, too. My son has one and is only allowed to play it in the car - makes it special. And, boys are not the only ones who like Pokemon, around here, anyway. The 8 year old girl next door has more stuffed Pokemon animals than my son, and lots of the cards and toys.
As for traditions - start a few that are totally your own. At this age they understand the special meaning, and are very good at coming up with their own ideas - she may surprise you with what is really important to her. One thing my Dad used to do when we were little, and I still do with my son, I think I will be doing until he marries and moves out. I think someone mentioned a snowglobe, and this is what my Dad bought each of us every year - just from him. He would use a different color yarn for each of us and attach it to the snowglobe package, then wind the yarn through the whole house - up and down stairs, in and out of closets - and tie it to our beds. Then when we got up Xmas morning this is what we had to do first - unwind the yarn until we got to our globes. It wasn't until not too long ago that my Mom confessed that this was a "secret strategy" to keep us occupied while she and Dad had that first cup of coffee and got ready to face the day.
Congratulations, and enjoy !!