US 20 across Ne. is quite scenic through the sandhills and along the Niobrara River. 385 N out of Chadron ,Ne is a fine route to bring you to the Hiils, then stay on it through Hot Springs to Custer City, then e. on 16/16a into CSP, or continue n. on sd79 from the junction of 385& 79 to sd 36 direct into CSP.
MAKE RESERVATIONS for a site in Custer State park. Do not 'wing it '.
Visit Badlands NP on your way back east out of the Black Hills. Use sd 44 to Interior,SD, then back up through the 'Wall' to I 90 E.