X.O.
The Safe House (spy-themed restaurant) in downtown - kinda a neat place, but would probably be more enjoyable with a group
downtown Milwaukee has a river with some really nice restaurants along it. I don't recall the name of it, but there is a British-themed restaurant with riverfront seating
You could hang out by the shore of Lake Michigan. Not sure if it is walking distance from your hotel--I used to ride my bike there from Marquette's campus.
The Milwaukee Art Museum
Marquette University has the oldest building in North America - the St. Joan of Arc chapel which is a tiny brick chapel in the middle of Marquette's campus. They also have an art museum on campus that is free to the public. Marquette is just a few blocks outside of downtown
There is a mall right on Wisconsin Avenue; Water Street has many restaurants and bars
Really, just walking around the downtown area is enjoyable. But, beware, there are MANY panhandlers, so don't be shocked by that.
ETA: If you're at all into architecture, there's a beautiful Orthodox church designed by Frank Lloyd Wright in Cudahy, which is the town due West of Milwaukee--you could catch a bus there. If I am recalling correctly it is Ascension.