I have an idea...dedicate the open space to the local youth athletic organizations. I read in previous Daily Local articles how soccer clubs in Westtown balked at paying fees to the township...This ultimately led to kids not playing as much soccer as they could have. How about doing it like West Bradford does with most of the youth athletic fields?
In West Bradford, fields are either 'dedicated' to the local youth athletic organization (WBYA, linked to the side), or, actually leased for a nominal fee. WBYA is then responsible for all maintenance costs and scheduling on those fields. The residents of the township are able to utilize the space when there are no games or practices, which is the preponderance of the time. This keeps plenty of green space that will not be developed and keeps the costs down to the residents of the township!
Back to the original article, you'll notice I placed quotes around the word never...this is because no lawyer has ever met a contract drawn up by another lawyer that couldn't be broken. Easements are great, sure, for a while. Dedicating the land to local organizations (and yes, I am partial to the youth, but how about birders? hunters? Historical Societies?) seems to me to be a better way to do it.