I ran teh entire season with the Milwaukee Region SCCA and WAI. You can check out
http://scca-milwaukee.org/ and
http://www.waiautox.org/
Both are great groups and almost all of the events are at miller park.
Don;t just watch, come and run, you'll be glad you did. If you MUST watch, you can come on any Sunday and check it out, you do have to sign a waiver though if you plan on watching.