We did a 48 hour hopscotch-a-thon, and I don't recommend it. It gets cold and people get tired.
One fraternity had a hot tub-a-thon. The way it worked was similar to what Tom described. They set up the hot tub near the union where everyone walked passed and basically they asked (begged) people who passed for money. Whenever one GLO would host a philanthropic project at my school, the rest would support it with a donation. Another option is to allow members of other groups to get into the hot tub for a price. You could also encourage professors to get in on the act.
