Last winter I used my son's L.L. Bean school bag. It was comfy, big enough for me to wear and large enough to carry 4 sweatshirts and 4 rain ponchos (ok they weren't opened yet, and once they were we had to wear them) and carry some treats around in them. I am looking forward to spring, when we won't need the sweatshirts in the morning!
In general, any good book bag or day pack that you would use would work great. I would avoid the plastic ones, they just aren't very comfortable.