I like the Great Lakes museum and submarine tour at Manitowoc.   You can actually stay overnight on a WW II submarine!

You can continue north up to Door County or over to Bayfield and the Apostle Islands, across to Duluth Superior harbor then south along the Mississippi Great River Road....

Also maybe take in a race at Road America, a very interesting track. I enjoy going to the Indy car races there. 

Lots of things to do!