If you go back across northern Nebraska on Highway 20 there is a fantastic fur trade museum in Chadron. Could be the best on in North America. It's better than the one in Ottawa, I know for sure. Also, you could spend some time in Valentine and rent some canoes for a day and paddle one of the nicest rivers in the US, the Niobrara. Good camping at Smith Falls State Park. You could camp in the park and then paddle down the river from the wildlife refuge put-in to your campsite. Nice day's paddle. Kids would love it if you have kids with you.
Personally, I'd far prefer driving highway 20 than the interstate across southern South Dakota.

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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