Glad you got to see our great state...

I've never done any of the 'sand hills' rivers in that portion of the state...  hear that they are great.

One of the most challenging is said to be the "Dismal River".  Great name for a fast moving river!  It is in the heart of the sand hills, about 70 miles south of Valintine.

Niobrara is said to be one of the most scenic, and easiest to canoe.

Any of the 3 Loup rivers (north, middle or south) are great.

Another old favorite of many of local paddlers is the Cedar river.  This is on the east edge of the sand hills north west of Columbus.

Anyways, hope you had a great time, and return soon.