Summerfest is the granddaddy of all Midwestern festivals and the largest music festival in the world (so says the Guinness Book of World Records). For 11 days in late June, top national musical acts (as well as unknown college-radio mainstays) perform on innumerable stages along the lakefront [1], drawing millions of music lovers and partiers. Agoraphobics need not even consider it.
Shop around for discount coupons at grocery stores and assorted businesses, or consider a multiday pass, available at businesses all around town.
Links:
[1] http://www.moon.com/destinations/wisconsin/milwaukee/sights/downtown/lakefront