Madison -- DellsEric's opportunity is pretty tempting. It sounds fun and simple for set up. Otherwise the suggestion to go to Wis Dells in late may works for me. I'd prefer to avoid the Holiday weekends as people may have family gatherings then.
Close together. Use the Madison event as a date, and use the Dells as a home base for a midwest event. Less than 45 min away with two Highways to chose from (one more scenic than the other, and there is even an option to take a ferry on a more scenic route.