Door County
Tucked away Northeast of Green Bay, this area is a tourist mecca for Wisconsin. Be prepared for long, SLOW drives up into the peninsula, and even slower return trips. Quite a few lighthouses there (see section below) along with many quaint villages. Local wines and Fish Boils are popular. I've never been there, but Washington Island is also a popular destination.
Lighthouses
There are three lighthouses on the Door County peninsula, not including the two in Sturgeon Bay. If I had to pick one to see, go to Cana Island. The western shoreline of Lake Michigan is dotted with lighthouses from Green Bay down to Kenosha.