Depending on the dates of your trip the world's largest live musical festival takes place in Milwaukee along the lake each summer.
http://www.summerfest.comAssuming you want something other than an ordinary hotel, I suggest staying at County Clare. This small pub and inn is on the city's popular eastside close to the lake and within easy walking distance to the riverwalk and associated night life.
http://www.countyclare-inn.comThe Milwaukee Public Market and the area surrounding it is the best bet for a quick meal followed by drinks outside at one of the neighborhood bars.
A tour of the Harley-Davison motorcycle museum is a lot of fun. Thursday nights thousands of motorcycle riders converge to listen to live music. Despite some rough appearances it is an incredibly friendly crowd.
The zoo is world class and a great spot if you are bringing children along.
Since this is my home I could go on and on with things to do. If you let me know you interests I would be happy to tailor my suggestions.