Old World Wisconsin near Eagle, WI is a great living history museum. It is just west of Milwaukee.
The Milwaukee Zoo is outstanding and is totally wheelchair accessible.
The Experimental Aircraft Association just south of Oshkosh is worth the stop if you are interested in private, military, or commercial planes.
In Manitowoc there is the Wisconsin Maritime Museum. They have a WW2 submarine, the USS Cobia, on permanent display. I believe everything at the museum is accessible except the sub.
Being from the midwest, I assume that you are a Green Bay Packer fan so you could visit Lambeau Field.
Just a few things to take into consideration.
BarabooBob