The answer is to stay out of it! The house, I mean.
Six months ago we left the apartment in The Netherlands in super neat and clean order. Today when we returned, we had very little to do to make it livable.
Somehow while we were gone the small fridge/freezer had defrosted and now needed demolding. But it did not smell bad. In fact the small bowl of coffee grounds that I had left on the fridge shelf was moldy, but it still smelled like coffee. So before we threw in our groceries, I scrubbed the whole interior of the fridge and got out the mold and turned on the electricity. I think that I know the problem. After we left in April, someone cut off the electricity to the little icebox and probably closed the door. Then someone else came along and saw the mold and left the door open. That will teach us to next time leave notes on all appliances that can experience damage from well-doers.
We also experienced water staining in the toilet bowls. I can't wait to try out soda and vinegar solutions like I read about on Pinterest. If that does not work, I have other methods.
But in general, the apartment looks great. A week from today it will look reasonably empty. More on that another time. Maybe even with photos.