If you play games to kill time or relax, then casual games are what you’re looking for. These are the games you can play while you’re on the bus ride to work. Playing games during their down time, the casual gamer, like you, takes it in stride.
The main two factors you should consider are price and entertainment value. A game that is picked up and put down frequently needs to be easy to get into, but have the sustaining power to keep you amused for hours for those longer sessions of down time. Many casual games allow you to download a demo or trial version. Check them out and find something you enjoy.