I've wanted to hike the New York Highline since I first heard about it. I should go back when it's warmer, of course, but I still thought it pretty cool:
The Highline shows that an elevated urban park can work, both as public space and as a great way to preserve historical (or expensive-to-remove) infrastructure. I hope Chicago's Bloomingdale Trail follows the same model, once the city sees fit to authorize it. (The Bloomingdale Trail umbrella organization has comparison of the two projects, about half-way down.)