If you're driving, Ashfall National Monument (or something like that) is in NE Nebraska. One of the coolest fossil sites I have ever visited. A volcano went off in Idaho about a million years ago and sent a shower of ash down on a watering hole in eastern Nebraska burying tons of now extinct animals. You can see them in the excavated watering hole
Just NW of there is the Niobrara River. A National Wild and Scenic river. You can rent canoes and go on one day or multi-day trips down the river. River has current but requires little skill so novice canoeists can navigate it. One of my favorite rivers. The town of Valentine is the center for canoe liveries.
N of Valentine in South Dakota is the Pine Ridge reservation where you can visit the site where Native Americans held off the federal government in a modern indian war in the 1970's.
On the south bank of the Niobrara, much further west, just east of Gordon and only accessible by the river or a 4wd is a curious site. A small monument dedicated to the US Cavalry which burned a wagon train which was illegally heading into Indian territory. Probably one of the few instances where the US armed forces acted to protect Indian lands from white encroachment. I guess it didn't have much effect in the long term.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13