You know, Metal Gear Solid: Portable Ops plus is a great pick-up-and-play game. It's not casual, but the format of the game makes it very easy to pick up and put down whenever. You essentially play as Snake (and pals, if you have a MGS: Portable Ops save file on your system) and you proceed through generated stages with some random elements trying to recruit new members to play as, or to assign to different development teams, as well as pick up various weapons, items, and books which will permanently boost the stats of a soldier. It's a great game you can easily sink a hundred hours into, or at least, I sure did.