I'll toss you a list of games, and a very brief description.
Banished Town builder. Start with a handful of families and limited supplies. Grow your village so that you can survive and thrive. Very relaxing.
Creeper World 3: Arc Eternal Siege warfare. Minimalist graphics game where you try and stem the tide of creeper until you can push close to a spawner and destroy it. Here's the free flash version.
Diablo III ARPG. Don't play hardcore and you won't have to care when you fall asleep while running into walls.
Gemcraft: Chasing Shadows Tower defense. Summon and combine gems with various effects while completing tons of different levels. Level up and put points into skills to gain mana faster or increase gem strength, etc. Also comes in a free flash version.
Gnomoria Town builder. Dwarf Fortress + actual graphics. Take 8 gnomes and build a town that can grow to accept nomads. Dig deep to mine new materials and gems. Forge weapons, build homes, and try to survive against the goblin hordes (optional, you can turn off fighting.) Super fun and relaxing game.
Just Cause 2 Open world TPS. Run around and shoot stuff. You can grapple onto just about anything. You also have a parachute which you can magically deploy at about any time. If you ever played Mercenaries for PS2, it's similar. Mindless action.
Kerbal Space Program Space sim. Build a rocket, pray that it doesn't explode and that you remembered to put all the parts on it. A little bit more engaging than the other games, but it can be very relaxing to build a rover and drive around some of the planets.
La-Mulana Just kidding. Fuck that game.
Legend of Grimrock RPG/Puzzle. You move as a party of 4, trying to escape from a dungeon. There are monsters, traps, puzzles, and secrets. Most of the puzzles are pretty easy. It's finding the secrets that takes work. They're optional, but they offer some good loot. Battle is in real time.
Orcs Must Die! 2 Trap-based tower defense. Play as a wizard-type in 3rd person. Lay traps for Orcs. Upgrade traps or equipment. Pretty mindless most of the time, but entertaining nonetheless.
Sanctum 2 TD + TPS. Pick a character, weapons, and up to 4 towers. Stop the enemy from reaching your core. You fight the waves of enemies while inside your maze of towers.
Sid Meier's Civilization V Turn-based 4X. Start as a fledgling nation and explore, expand, exploit, and exterminate (4X) your way to victory. Tech ranges from basic melee to giant death robots.
Space Engineers Sandbox. Build whatever you want, but in space.
Space Hulk Ascension Turn-based tactics. Adaptation of the board game Space Hulk. Lead small squad(s) of Space Marines through the wreckage of massive warships infested with aliens.
Torchlight II ARPG. Choose a class, kill mobs, get loot. Good variety of skills and equipment unlike Diablo III.
Waking Mars Platformer/Puzzle. Explore the underground caverns of Mars, learning how life is surviving beneath the surface and opening new pathways by completing various puzzles that revolve around the various lifeforms.
X3: Reunion Space trading/combat. Pilot various kinds of ships from small fighters to massive cargo or capital ships. Trade goods between various systems to gain reputation and credits. Buy bigger ships. Good for playing while listening to a podcast in the background.
Zombie Shooter Shoot zombies.
I have tons of other games, but I think these are the most relaxing. Hopefully some of these are of some interest to you!
Edit - Markup was a mess. Still learning!