If she doesn't really need anything maybe you could take her to a nice restaurant or to the theatre?
Or you could actually take her shopping and let her choose something, that way, she gets what she wants and also gets to spend some quality time with her grandchild.
Supplement from 07/11/2008 04:08pm:
Just had a another thought, you don'tsay how old you are or how old your nan is, but how about some IOU's? make a little book and write things like...i promise to weed your garden for an hour, i promise to hoover once a week for a month, i promise to get your shopping, i promise to wash your windows. that sort of thing, i did that for my nan many years ago, she loved it, as it also meant she saw me more often and she got all those little jobs done that always get put off