There are so many possiblities
The river is 800 miles through montana alone.
lots of great camps.....look at photos in "place2paddle" here on pnet and the trip reports for the missouri.
send me your email and i'll send you plenty of photos.
How many days you want to go?
Any boat ramp is a good place to start.
The WHite Cliffs is the most scenic flatwater in the lower -48.
Google photos for "White cliffs of the Missouri" or something to that effect if you want to see photos.
Also look at the two website of my friends who paddled it.
http://www.charitytravelers.com (i think these are the urls.