The post office is pretty short, maybe they could parkify its roof.
But I think more useful than a big destination park would just be more "placemaking" areas with public seating, scattered all around so you're never too far from a place to rest. Some areas are experimenting with filling in a few curb parking spaces with little patches of green and some cafe tables.
But there's probably none of these things yet because there's nobody to ask for them. I don't even know what the river north community organization is. All I hear them doing is Taste of River North. SOAR seems like the most vocal and proactive of the downtown neighborhood groups working to improve or modulate their environment.
Edit: speaking of parks, the commenters on this Curbed thing about upcoming parks have some... interesting ideas about what the city should be doing with the money instead.
http://chicago.curbed.com/archives/2...forward-to.php It's so weird to me to see people opposed to park improvements or parks in general. As if we had one fewer park we could use that money to cure gang violence and homelessness.