I have three options for a game I pick up to relieve stress...
The first has been mentioned before: A game with messy(ie your mowing down enemies) combat set to a low difficulty. My personal choice for that tends to be Ratchet: Deadlocked. Nothing relieves stress like tearing through a graveyard of zombies(ROBOT zombies!) with a wrench, a pair of pistols, a giant electric flail... And a missile launcher.
The second choice is something just generally fluffy and fun, maybe something sans combat like a collection of mini-games, a simple platformer, or one of those 'objection-less' goof-off games like Electroplankton. As of late I find the Cooking Mama games very relaxing and fun myself. You also get the joy of making yummy food... With out the resulting 'ugh this is gonna make me fat' guilt afterwards. I also like goofing off in the minigame parts of Kingdom Hearts, cause if the combat or minigames don't work I can look to the other!
Lastly I also like to play a game with characters and a story that I enjoy and just continue on with the story, sometimes it helps to just get my mind off the worries of one thing and into the world of game. I most often do this with one I've played before, the familiarity feels good and I get to revel more in a world that I enjoyed before.
