Check your bylaws.  Unless your organization is a private foundation, it would be very unusual to have a bylaw provision that provides a “prohibition” against attempting to influence legislation, or “lobbying.”  501(c)(3) public charities are permitted to lobby so long as it is not a substantial part of their activities.  A provision prohibiting a “substantial” amount of lobbying is often included in the articles (certificate) of incorporation and/or the bylaws.  (See Ready Reference Page:  “Articles of Incorporation Establish Basic Form of Nonprofit Corporation”)

Unfortunately, a lot of people don’t understand that public charities can lobby and some put the effective prohibition that applies to private foundations into public charity bylaws, thus imposing an unnecessary, and, in my view, a terribly self-defeating restraint on their activities.  If you are a public charity, you are allowed to lobby so long as it is not a substantial part of your activities, and your bylaws ought to reflect that opportunity.  If they don’t, you ought to change them.

Whether seeking to establish a county-wide park district would constitute “lobbying” depends on whether it would require a new law, perhaps a new state law or a county ordinance, to accomplish.  If it could be done administratively, perhaps simply by reorganizing the county parks department without legislative authorization, it would be outside the federal tax definition of lobbying.  (It may be within a state or local definition of lobbying for lobbying registration purposes, however, so be sure to look at those rules if you might have to register locally.)

If the new park district can only be created by legislation, promoting the concept would be lobbying under the federal rules.  But a public charity is specifically permitted to lobby, and if the issue is important to you, you ought to get involved.    (See Ready Reference Page:  “Lobbying Rules Create Opportunity for Charities”)  As I have often said, a public charity that isn’t lobbying is probably not doing its job.