Arts and crafts for 9 year old girls are usually good- I work for a toy drive and that is what most girls that age request- of course you get the few that ask for sports equipment etc but typically it is arts and crafts.
Crafty type things are great.
Going to agree with the arts and crafts as others she said. My 9 year old daughter loves arts and crafts, one of her favorites is bracelet making.
Mama to a 9 year old here! I agree that the arts and craft kits are a big hit with most girls this age: make your own necklace/bracelets, sand art, make your own headbands, etc.
Like PP's craft stuff is very popular.
I definitely agree with art supplies, and love the new backpack idea!
Yes, YES and another YES to arts & crafts.
I JUST found about this new type of YARN last night, that is very different than the usual. It's called LOOPS Yarn. It is made in such a way that the loops one usually has to make with knitting needles or a crochet hook, are already stitched into the yarn. So one just does FINGER knitting. NO knitting needles or crochet hook necessary. It is great for kids to do on their own, with minimal instruction. So, OP, the little 9 yr can make a scarf or infinity scarf to go with her new backpack if you choose coordinating colors.

(2 balls of yarn should be enough to make a scarf for her.)
The yarn has actually been out for a year, but I hadn't heard about it before. Doing research last night, I've learned that many, many people have already made so many projects that go really quickly. A few crafters have even gotten their DHs hooked and are doing it while watching their sporting events on TV.
It "knits" up easily as one doesn't have to wind & form the loops around needles. All that is already done. One just has to lay two rows of loops back to back. And depending on if one pushes the front loop through the back loop, or pushes the back loop through the front loop, determines if one is making a "knit" stitch or a "purl" stitch. It's that easy!
Here's a video of it. They are all doing the "knit" stitch where it's all smooth and the stitches look like little "V"s. Towards the end, when you see the bottom of the scarf, on the right side there are 3 raised rows, those are "purl" rows. Yet, once learns the two stitches, one can put them anywhere and can create her own design.
Here is a video with detailed instructions on how to push the loops and form both stitches. Then Kirsten shows how to bind off the end of a scarf (the same way we used to go around the edge when making a potholder on a loom.) DONE! Kirsten then goes into more detailed stuff that is normally so complex with regular knitting. But the 2 basic stitches and the bind off are all a little girl really needs. Shown that, she can make her own scarf any way she wants.
My suggestion, if you get the loops yarn , (JoAnn Fabric has a better selection than Michaels right now. The Loops Boucle yarn has been discontinued there,) is to include a paper sheet of written, basic directions of the two stitches and the bind off, in case they don't have easy Internet access, WITH the these three links, so an adult can go over the 2 stitches with her and later the bind off.
This link has the easiest to follow
written directions (with pics included.)
http://www.myhobbyiscrochet.com/2018/10/strawberry-waffle-loop-it-scarf-free.html
I'm excitedly waiting for JoAnn Fabric to have a good sale on it,

as I want to get 8 balls to make a soft, fuzzy afghan with the yarn, it works up so quickly. I may do one with this "basketweave" stitch in which one just criss-crosses every two front loops and just keeps going along in a straight line with the back loops. With regular knitting on needles this is complicated!