You want to drive one day but be a passenger the next, not a problem with Wunder. If you want to drive you must set up your primary role as a driver first by going to:
Account > Travel Details > Primary Role.
Then you can set your car model, registration and seat count.
To set up individual days to be riding as a passenger you can go to:
Account > Travel Details > Weekly Routine
You'll be able to change individual times, and set your role as either a driver or a passenger. You will also be able to change the Weekly routine settings by pressing the Calendar icon at the top of the Trip Feed.
Please note that you can't change your role if you have trips planned with other users. You must cancel these requests or offers first.