Something she wouldn't buy for herself or something that she really, really likes.
Examples - my daughter knows I love books by a certain author but I usually get them from the library and won't buy them - so she bought me one. She also knows that I love tunes and bought me an I-tunes card so I could pick what I wanted.
My mom knows that I love to go out but rarely do because of the expense, so she bought me a gift card for a restaurant that I love.
She also knows that I love books and would not buy them for myself but she doesn't always know my tastes, so she got me a B&N gift card so I could pick what I wanted.
My son and daughter-in-law have bought me stylish coats that I would not have thought to buy - or give me a gift certificate at Macy's so I can go get which one I want.
Those are just some ideas - real and gift cards - from an old mom who has kids 30, 32, and 34. It's kinda cool because I never know what I'm getting!