Write a family history together. This one combines the gift of your time with writing, plus you'll end up with something the entire family may treasure.
Leave notes for her. Write little notes about all the things you appreciate about your mom and hide them around her house.
Take her to dinner. Make a reservation just for the two of you. And make sure it's special, whether it's a fancy restaurant, a picnic with a beautiful view, or someplace tied to special memories you both share.