The Harry A. Miller Club rents the Milwaukee Mile every year for a 2 day event. See
http://www.harrymillerclub.com/index.asp.
Besides Miller & Dusenberg Indy cars from the '20s & '30s, over the years there's been lots brass-era cars like Stutz, Franklin & Mercer as well as GP cars from the '20s & '30s like Bugatti & Alfa Romeo. A recreation of Barney Oldfield's Golden Submarine. The restored Mormon Meteor. A Novi. And post-WWII Indy & dirt cars through the late '50s.
Lots of these cars are from riding mechanic periods. I got lucky enough to get a ride in a 1927 Amilcar a few years ago.
Besides the car owners & their crews, frequent attendees include A.J. Watson, Sonny Meyer (Louie's son & long-time race engine builder himself) & Ron Hoettells, the developer of the Sesco midget engines.
Paid admission gets you into the pits all day. The event is low-key and spectator friendly. When cars aren't on the track, they're usually in the pits with the hoods up for inspection and photo ops. The owners and crews are up for talking about the cars.
Seeing (and hearing) these great old cars running on a historic track is special. Check it out if you can.
Andy