After viewing you map, I thought I would suggest the Crazy horse monument, it is not to far from Mount Rushmore, It is much larger than Rushmore and very much a work in progress

http://www.crazyhorsememorial.org/

Another thought, Devils Tower is not far off I-90 in WY.
http://www.nps.gov/deto/index.htm

Also, If you are going to Fort Niagara. I would recommend crossing the border at the Lewiston crossing on I-190/Ont 405 unless your going to backtrack to the Canadian side of Niagara falls. The traffic is much lighter at Lewiston and the crossing is always been quicker in my experience.