Might be able to get away with RTS games, there is violence but you see very little of it being high above the combat. Will take some convincing though.
Sins of a Solar Empire is an easy option if your into strategy.
There are city builder games like Sim City 4 (of you want to brave the server situation) or Tropico 4 (Think a Cuban dictator simulator 😀).
Aforementioned Valve games and Portal series, they really are very good, no violence games that looks very nice.
Definite +1 to Indie games, they wont push the graphics card in any way but are very good and often have great aesthetics. I suggest looking into Braid, LIMBO, Machinarium, Super Meat Boy and the BIT.TRIP series. You might even be able to buy Humble Bundles off people...