Fort Lincoln is next to Bismarck, on the Missouri river. Gen Custer stayed there. Last place he lived, left there for Little Big Horn. His house and several other buildings have been reconstructed. It has a nice campground and is worth the visit.
North of Bismarck about 40 miles is Fort Mandan. Lewis and Clark stayed there one winter. The reconstruction of the fort has been done really well.
Fort Union is up by Williston on the confluence of the Yellowstone and Missouri rivers.
Check the states websites, they have several events at all three places that are really interesting.
While at Teddy Roosevelt south unit park be sure to take in the pitchfork fondue and the musical.
Between Dickinson and Bismarck is a highway called the enchanted road. A man from Regent has built huge steel structures about outdoor scenes. Runs about 30 miles north and south.
There are many place along I 90 and 94 through Mont on the way to ND, oldest prison in Deer Lodge MT, the start of the Missouri river by Livingston, Little Big Horn.
Go east through MT on the Sothern route to Bismarck up through Ft Mandan, over the Garrison Dam. Then west to HY 85 then north to the north unit of Teddy Roosevelt north unit. Up to Ft Buford and then west on Hy 2.