Depends on what you mean by relax. Do you mean "no effort gaming" or "slow moving gaming" or "soothing gaming"?

No effort: Grab any game and put it on 'Easy' mode. Seriously, it's actually kind of therapeutic to mow through a game like that once in a while, but don't do it often or you'll get incredibly bored.
Slow moving: Anything turn based such as the Tactics games of the FF series and, of course, the Civilization series. You can do everything in your own time that way.
Soothing: I second Flower.