Potholders are easy and a welcome gift for most moms. As ComfortB says, pillow cases are easy.
A basket with microwave popcorn, gift coupons for McDonalds, two candles and dollar store candle holders and a DVD and call it "Dinner and a movie."
Or if she sews fill a pretty basket with sewing notions, pins, thread, needles, a pattern.
Or for a gardner, a basket with seeds, gardening gloves, small shears - things like that.
Basically look at your mom, think about the things she loves and enjoys doing and make her gift all about her and her likes.