All kinds of things of national merit, including historic sites, national monuments, national memorials, historic trails, national recreation areas, and, of course, the national parks. All of these are run by the NPS, part of the Department of the Interior. The NPS has some places that are run in conjunction with tribal agencies.
This listing does not include the wilderness areas, national monuments and other protected areas run by the BLM, Department of the Interior. The largest portion, though, is simply public land, much of it leased out to ranchers and others for cheap.
And, of course, there are the national forests, run by the US Forest Service, part of the Department of Agriculture.
:)
Lynn