When I taught in South Dakota I traveled through this area a lot back to Utah. I've only seen it in passing, and have never stopped for any number of days, but I do have my impressions of the area.
In all the route between Brookings, SD, and Provo, UT, US83 between the South Dakota border and Thedford, NE, were always the prettiest and most interesting, no matter what time of year.
I would loved to have spent more time in the Sand Hills, shooting the wondefully remote, cow-pathed and green hills in the Spring. In the Winter they were all together different, more forbidding, but the way the cloud-diffused sunset played on the hills was glorious.
I also wished I could have taken a bit more time with the two bridges just North of valentine. They crossed quite a gorge there, and one bridge gave an excellent view of the other.
Here's one of the very few shots I took in the area: http://chem.dynu.com/photo/photo.asp?Photo_ID=139 very early in my photographic training.