I guess it also depends on your definition of "open space". If you're going to count ball fields and parks as "open space", then the options open up quite a bit, as begreen already mentioned. My mind went to preserved farm land / wild space, far preferable in my mind to any over-trafficked park or ball field, but perhaps that's the best compromise that can be pursued, in this case.
Your township likely has an open space preservation program, which is almost certainly under-funded for that sort of land purchase on their own, but would also likely be staffed by a few people who know how to tap into state resources. My neighborhood is wrapped on 3 of 4 sides by parks, one of which was donated by a wealthy neighbor, one being state land managed by the township, and the third being land the township purchased from a local farm. There are many ways to skin that cat, although other than showing up for township meetings and voting on the approvals, I'm not involved enough to give any direct guidance.