The other day Dennis was showing us pictures of his cross-country trip and told us about this park in Seattle. It's an urban park built under the I-5 Colonnade, a great reuse achievement which could work in any city; possibly they got the idea from Burnside Skatepark. The park looked really cool but I didn't get a feel for the true size of the park until I saw this video over at twofivefix.
Although they make mention of the crazy twisting teeter-totter you don't get to see it. But you can check their wiki-like page for more photos and info here.