I just did this really quick, but I have been thinking a lap around Lake Michigan would be really cool. Time it to run through St. Ignace during their summer classic car show (June I think).
This map I laid out on Google Maps shows 1014 miles, but would need a ton of fine tuning to keep to the lake shore as much as possible. Could make a great two or three day event.
St Ignace, MI to St Ignace, MI - Google Maps