I found the Havre Beneath the Streets tour (Havre, MT) fascinating. Reportedly, the Wahkpa Chu'gn Buffalo Jump there in Havre is also exceptional and unusually well-preserved; I didn't have time to visit it myself when going through the area.
The Nelson Reservoir campground, a spell to the east on Route 2 between Malta and Saco, is a pretty public campground with no hookups and minimal amenities, but also available free of charge. It's on the shore of the reservoir and has some trees and just generally pleasant surroundings.