Be one with the river in your very own kayak. Here you'll get some of the best views of the city, and get in a pretty decent workout too. Of course, it's is seasonal.
If you love the great outdoors, get yourself to the 606. An elevated jogging/hiking/biking path that takes you through some of the 312's most ecclectic neighborhoods like Bucktown, Wicker Park, Logan Square, and others.