How about service gifts then? Take her out to a movie & a "khaki pants" dinner (our boys don't go out to dinner in jeans unless they're wearing a dress shirt-makes it more special that way, they usually wear khaki pants & a top). Go with her to get a pedicure or manicure, if she's a reader, spend a kidless day wandering through a bookstore, buy a cup of overpriced coffee, sit & read a few passages in some books until you find a couple of perfect books.
One sister's family is like that-they've got it all & if they don't, they go buy it. Makes it very hard to shop! I've found w/my niece (25 & still spoiled as all heck) she likes homemade stuff-scarves & hats I've crochted, I once made her a journal (bought a nice covered blank journal, drew a picture on the front & wrote an inspirational quote on every other page at the top), I've drawn the Eiffel Tower (painted regular printer paper w/red acrylic paint, drew the Tower on it, wrote a passage about it in French & framed it in a $2 frame that I painted black-it hangs above her TV in the living room), I've made stationary on the computer for our nieces before... email if you need more ideas.
Good luck!