Fort Assinniboine sits a few miles southwest of Havre and works best as a seasonal add-on to a history day. Established in 1879, it was once a very large frontier military post, with surviving brick buildings now interpreted through tours.
The history is not simple. The fort was part of the U.S. military presence on the northern Plains, and later its lands became tied to the Rocky Boy's Reservation and agricultural research. Go for the scale of the place, but leave room for the harder context.