J.C.
This isn't close to the NW side, but I hope it helps. There's one in the south loop at 14th and wabash, it's right across from a Buddhist temple where 14th doesn't go all the way through. Be careful, it's a nice park (Colliseum park maybe?) but when we went there last time there was a really weak board on one of the trains.
Hope that helps a little!
J.