In the Black Hills area there is Custer State Park with Buffalo herds and Prairie Dogs, worth a day trip. A day for Rushmore. And the third day for Wind Caverns. That leaves out the Badlands NP, Wall Drug and the Minuteman silo. But those can wait for the next trip.
You can spend 60 days in Yellowstone and not see it all. If you don't hike then there isn't a lot to do at Grand Tetons except drive to different view spots and take the ride across Lake Jenny so I'd leave your division of time as is. There are several different hot spot areas in Yellowstone you can spend a day each on.
On your trip up or back you may want to stop at Devils Tower since you'll be close to it when going between YNP and the Black Hills. It's a 2 hour visit which will give you time to walk around the paved trail around the tower.
Better make reservations for YNP soon and maybe for the Black Hills too. Custer State Park is a very popular camping spot.
