It's hard to find time to volunteer at Wiscon because most of the volunteer jobs take place when something else you want to attend is going on. So I don't do it every year, but here are some of the jobs I've done:
Hanging art at the art show. Some of the artists mail in their art and volunteers hang it. I enjoyed this because I got to decide how to display the art (although some artists include a diagram of how it should be displayed).
Con suite. It's fun to be one of the people in charge of cooking and serving hot dogs and pizzas because you meet lots of people who are happy because you're feeding them free food. :) And you get to overhear interesting conversations.
Packet-stuffing. A fun, loud, apparently chaotic but actually very organized event. This year at one point the volunteer coordinator came in to see how it was going and said we would get TOTE BAGS as volunteer gifts. We all went "OOOOOOH."
Cashier at the art show. I found this challenging because I have not done a lot of cashier type work before and there were a lot of things to remember about how to keep track of whose art was being purchased. But I was working with a couple of more experienced volunteers who were very supportive. The show was relatively quiet during most of my shift and artists were dropping in to see how their work was selling and to talk to each other about the challenges of making and selling art at shows like these—I found that very interesting, and I easily learned as much as I have in any panel.