The GTA games are very well suited to casual play. They're divided into lots of relatively short missions, and in between them you have the run of the city to do whatever you feel like. Drive a car around at breakneck speed, find collectibles, customize your character, etc. It's also not very skill based, which is another point in casual gaming's favor. There are some challenging things, sure, but the game's fun is mostly about having the run of a city and doing whatever crazy stuff you can think of. The stories are usually pretty good too.