we also just got back and took the same advice as RN.
stayed away from keystone but took the drives thru the wild life loop and got right up to the buffalo.iron mountain and needles were awesome drives.went out to the badlands for a day,thats not to me missed.
the 600 mile drive each way, back and forth to Minneapolis is a hassle because its a long drive but there is no way to stop for the night because home would only be a couple hours more so you may as well push on and get it over with.

A few years ago I loaded up my bike (after the dig was over) and toured from Rapid City toward Bemidji,MN., where my brother lives. My route took me on secondary highways diagonally across the state through Pierre and ended in Sisseton, SD, because the weekend was approaching and there were bass that needed catching; my brother came down and picked me up. It was a delightful trip, staying off the interstate, sleeping in various state parks, and seeing the real South Dakota, which is in short supply along the main roads. I was glad to spend some quality time with my bro, but I was looking forward to biking though MN, as well.

Fly over country it ain't.

Crystal Cave & Angostura Reservoir are among my favorite spots in the state! Glad you enjoyed your time in SD. I have some vacation and a weekend to use up this year, sure would like to get out to the Hills before the snow flies.