Thanks guys!
Here is what I’m thinking:

Leave Seattle WA
Coeur dAlene, ID for 1 night
Missoula MT for lunch
Bozeman, MT to look around
Livingston MT for 1 night (and look around?)
Broadus MT for 1 night?
On to Custer SD, stop at Crazy Horse for afternoon
Stay in Custer SD 1st night
Custer State Park—most of the day
Mount Rushmore in afternoon?
Stay in Custer SD 2nd night
On to Badlands
Spend day at Badlands, including night sky tour on weekend evening
Spend 1 night at Badlands lodging
Head out to Mitchell or Sioux City SD to spend 1 night
End in Minneapolis

To ignorant me, it looks like I have 2 extra nights to play with the 9 I’ve allotted.
I feel like I missed good stuff in Montana, or maybe I’m not being realistic re how I’ve played this out.
Husband is considering skipping Broadus and heading for Custer, so that would give me a 3rd extra night.

Other notes:
• While I am trying the useful link sent by poster glc, I’m bringing a travel cot, toted in a golf bag, to use at the hotels that hear me say that I am a party of 5 people, but book me anyway.
• I read negative postings on tripadvisor that described a private concessionaire company that built ugly buildings with kitschy tourist shops to block the view of the Mt. Rushmore sculpture from the road so as to ensure the purchase of the parking pass. There were many more positive reviews than negative, but the negative gave me pause.

Here are my questions. Basically, where should I spend the extra 2 nights?
1. What do you recommend in Montana? Or where would you refer me to research?
2. Should I allot more time in South Dakota? How?