Katahdin Woods and Waters is a young National Monument without much infrastructure. Recent construction and cooperation with corporate & charitable sponsors has led to development of the Lunksoos campground in July 2021. I approached via the turnoff from Swift Brook Rd to Sebois Rd, before you reach the loop road, but Sherman Lumber Co Rd is another option. You may encounter logging trucks. The road is not in great shape, but passable.
A short walk from the East Branch of the Penobscot River boat launch, there are several free, walk-in sites. Two group sites allow nearby parking and provide tent platforms. The other 5 are on a small, walk-in loop through the woods; one is an accessible site. Vault toilets are provided. Sites come with bear boxes, picnic tables, and fire rings; fire permits are required (call Maine Forest Service 207-435-7693). Bring potable water or plan to treat it. Camping is free and there are no reservations. Midweek August I saw only 1 site occupied, but it's also a new campground. No cell service. Mosquitos and the occasional deer fly were annoying
You can enjoy fishing (license required), boating, hiking, stargazing, and wild-life watching(I saw fresh bear scat on the road on the way in; moose also frequent the area and maybe you’ll even spy a lynx!). There are Junior-Ranger programs, including a Night Explorer one. Drive the loop road to some short hiking trails and look for moose. The information station at Sandbank Stream campsite just before the loop road often has some brochures, including a bird list and interpretive guide to the loop road, though I would consider downloading them from the Friends of Katahdin Woods and Waters website.
If you’ve forgotten anything, stop in Millinocket or Medway to pick them up before you get into the Monument; it’s a long drive out and back over these logging roads!