Boy, this question hurts.
For anyone above a certain age with kids below a certain age this isn't a punishment or a challenge, this is A DREAM!! Heroes of might and magic 3, system shock 2, Anno (any version up to 1503), Morrowind, Civilization 3, Age of wonders shadow magic, Baldur's gate 1 and 2...
Whenever my wife finds the time she goes to her room to play Morrowind. She just got a new laptop and the first thing I did was install OpenMW and copy her save file.
It goes without saying that you couldn't finish any of those games in those 12 hours, except for System Shock 2.