Do you have a cute photo or 2 of your son when he was younger? Maybe doing something then that he either does now "for real" or refuses to do - pee wee football, cooking with a toy kitchen set, anything. Get a little frame or a collage-style frame that will take several photos, and stick them in there. I don't care how much money she has, she can't buy that. If she cooks and you have a favorite recipe (of yours or of his), you could give her that in a little magnetic thing that could go on a fridge - but don't do it if she doesn't cook and it would seem sexist. Those are things she can't buy, no matter how much money she has.
Is there a museum nearby that they might enjoy, or a decent live theater, and you could buy a membership or tickets to something? You could give them a "date night" and even get a "twofer" by giving it to her and him together, perhaps with something smaller for just her.
Personalized Christmas tree ornament? Someday she will have her own home and perhaps you could start a tradition of giving her an ornament every year so she will wind up with a collection. Craft shops have some unique things and you'd be supporting a local artisan.
Nice coffee if you have a local coffee roasting place - better than supermarket coffee and they support free trade initiatives in other countries.
Otherwise, go to a local bakery and get some fabulous brownies or pastries, have them wrapped individually (our local brownie emporium will cut their big brownies into quarters and put them in those little cups, so you can mix/match to give everyone a little taste of different flavors), and put them in a coffee mug with her initial on it. Done.
It's the thought that counts and hopefully she gets that.