Parks are an unusually public-facing government services—they exist as a service to citizens, for recreational purposes. Parks services want to promote their parks, to get people to visit them, so they have an unusually strong incentive to publish data. Helpful data each park’s location, amenities, services, fees, hours, address, entrance locations, and, when possible, geodata.
Trail locations, permitted traffic (foot, pack animals, bicycles, ATVs, snowmobiles, etc.), accessibility status (whether wheelchairs can traverse it), trailhead locations, services available at the trailhead, and a description of the trail. This really requires geodata to be useful.
OpenTrails, a project of Code for America, “defines a simple, common format for public trails and associated geographic information.” Even if not adhering to the OpenTrails standard, they’ve put a lot of work into determining what data should be included and how it should be structured, and should be used as a reference.