Originally Posted by CPMaverick

Agree, somehow I missed the Mt Cook tour. That's a lot of criss-crossing NZ. I'm a bit confused as to whether you are driving to CHC or not? Because you mention a bus, but then say you are returning the car at CHC.

Perhaps this would be a better option:

Day 9: Drive to Te Anau. See the caves. Overnight in Milford Sound, or Te Anau.
Day 10: See Milford Sound. Return to Queenstown. (Note, depending on weather you could drive straight to Milford on Day 9 and see the caves on return).
Day 11: Drop off car at ZQN. Take bus tour to Mt Cook Village. Stay in Mt Cook Village.
Day 12: Take bus to CHC. There is a bus leaving around 2pm and arriving around 7pm. https://www.intercity.co.nz

Alternatively you could self-drive but that would be 4 long driving days in a row. So either do a tour to Milford/Te Anau, or take the bus to CHC, IMO.