We've been across ND twice on I94 and can add a bit to the above good suggestions.
There is a nice Lewis & Clark Interpretive Center in Washburn north of Bismark. A couple of miles away is a reconstruction of
Fort Mandan, where L&C spent the winter of 1804/1805. A few miles north of Bismark is the site of the
Double Ditch Indian Village on a bluff above one of the few remaining free flowing portions of the Missouri River. Not much there except for mounds and depressions, but we really enjoyed walking around and reading the interpretive signs.
If you are coming across Montana on I94, I highly recommend a stop at
Pompeys Pillar National Monument. It's easy off/easy on from I94. A very nice Visitor Center there and in addition to Clark's signature, great 360 degree views from the top of the pillar.