Kettle moraine rocks, the blue loop will really give you a work out. Also there is a kickin Indoor Mountain biking park near wauwatosa (spelling?). I went there a few months ago and it is great. They have a bmx section, beginner mountain biking and expert. Its basically for small low speed technique. its called Rays Indoor Mountain Biking.
Check it out raysmtb.com
plus is has a foam pit and bike rental.