After breakfast, return to the Tower one more time for a little more hiking. I took the Joyner Ridge Trail, which starts northwest of the Tower and cuts a wide loop, allowing you see to appreciate the monument from a distance as well as traverse a variety of altitudes and terrains. It was beautiful, but fairly arduous, and took well over an hour.
Once you return to your car, return via State Highway 24 and US 14 to I-90, and thence to the Rapid City Airport for your flight home; the drive should take about 2 hours. If you have some extra time to kill, you could check off any items I listed under Rapid City or Keystone that you didn't get to in previous days.
I hope you enjoyed your trip! Safe travels!