As for the gift cards: make sure if you get one for a restaurant that it is one she likes and will use. There are people in my family that will not go to restaurants because food tastes better at home and you never know what is going on in the kitchen. Most people would like it, and might even like to try out a new place, but since you said she said she was hard to buy for, I thought I would mention it.
How about not buying anything? MAKE her something! A scrapbook, jewelery, baked goods, sewing project, or other personalized gift. Or DO something for her, like babysitting, wash her car, or give her a 'coupon book' for snow shoveling, lawn mowing, or planting some flowers in the spring.
Look at things around the house. Does she have plants? Maybe get her a low maintenance one. Does she enjoy cooking? How about one of those indoor herb gardens.
If you go the gift card route, the spa ones (mani/pedi/facial) ones sound great. Other ideas: if she likes to read, get one from Barnes & Noble or Boarders. If she likes coffee, get one from the local coffee shop. If she likes nice toiletries, get her one from Bath & Body Works or the Body Shop.