T.,
There are so many ideas you could with depending on what she likes. If she enjoys food, a gift certificate like others mentioned is a good idea or maybe you could make her a basket filled with a themed idea. Here are some I've done in the past:
-Basket of snacks. You can go sweet or salty or mix the two. I enjoy baking so once I made a loaf of banana nut bread and some cookies. I put both of those in a basket along with some hot chocolate (great for those cold days), coffee, hot tea, candies and some other sweet treats teh person liked. GOing with the salty idea could include things like chips, cheese & crackers, trail mix, nuts, etc...
-Pampering basket. I filled one of these with some bath products (bubble bath, a bat mit, lotion, shower gel, etc.), a bottle of wine, scented candle and a music CD with some relaxing music on it. It doesn't have to be expensive. You can burn your own CD, include an inexpensive but good wine, and get some small sizes of bath products for her to try. By the way, Target has a couple of lines of really good bath stuff that is not too expensive.
-I once made a basket for my mother-in-law of her favorite drink. She had once mentioned her favorite alcoholic drink that she orders when she sometimes goes out. She didn't know how to make it but enjoyed it. So I looked up the recipe on the internet and bought all the required ingredients and printed out the recipe on a small card and put it all in a basket with a scented candle, Jazz CD and a glass. She loved it.
-Scrapbook basket. My sister-in-law just got into scrapbooking and enjoying putting her kids photos and their trinkets in them, so for her birthday I got her a small scrapbook, some stickers, paper rounder and some fun papers and put them in a basket for her. I also went on the internet and printed out some ideas for scrapbook pages and put them together with ribbon in the 3 holed pages and made it into a little book that I included.
There are so many things you could do depending on what the person enjoys. Can you tell I enjoy making themed baskets? You don't even have to go the basket route, just find out what she enjoys and go with that.
BY the way, every year we draw names for Christmas with my siblings and my husband's. So it is a surprise who got who, we make each person fill out a small questionnare that asks what their favorite colors are, which stores they enjoy, their hobbies, wish list items and their sizes. This allows us to buy something nice for that person and we know it is something they will want and enjoy. Maybe you could think about creating a little questionnaire for her and get some ideas? Just an idea.
I'm sure whatever you do will be great! It's really the thought that counts.
-M.