I don't live there now, so I can't comment on how open or closed things are, but whenever we go back for a visit, I love going to the Milwaukee Public Museum. If they're open to doing tours, it's worth touring the Sprecher Brewery, who brew both beer and soda. And of course, there's always the Miller Brewery, but I've never done the tour there, so I can't comment on it.
In terms of food, make sure you hit a frozen custard stand - frozen custard is a super rich, creamy ice cream, and most of those places have really good burgers, too. Most popular ones are Kopp's and Leon's. Also, a buddy of mine has told me that Lou Malnati's now has one (or more) places in Milwaukee - absolutely killer Chicago-style deep dish pizza! And check out the Milwaukee Public Market - there's quite a few small restaurants in there that offer a wide variety of foods as well as stuff to buy. If by some chance they're open, check out The Safehouse, which is a restaurant/bar with a 60s James Bond theme to it.