Suggest to editors various feature stories they could run regarding your anniversary. Invite reporters and photographers to anything that is even the slightest bit newsworthy. And don't be disappointed if it seems as though the media isn't giving you the attention you feel you deserve. The media is inundated with countless press releases and requests every day, and they have to pick and choose. But they can't choose you if they don't know about your event.
History is written by the victors.
The fact that your park district is celebrating its anniversary means that you're a survivor. Times change, needs evolve, but you remain. You are an institution, a pillar of the community. As such, it's important that you chronicle your park's history. You can also find a gold mine of information at your local library and historical society archives.