This is my go-to lazy game.
TrackMania offers fast paced competitive twitch racing with a quick learning curve and a stupidly high skill ceiling, all in convenient (mostly) 5 minute bites. This is a game that you can leave running when you are busy with something else. Don't think that that puts it in the domain of casual gaming, though, as it remains a professionaly contested eSports event.
Nations Forever is the free-to-play piece of a larger puzzle. It allows players to race in the stadium environment ony and requires them to sit out every 6th event. Players are also not given the option of changing their car's chasis. This creates no competative disadvantage, however, as all cars in TrackMania behave exactly the same.
I often find myself browsing through my games library pondering which new and exciting game to start, only to launch TrackMania instead. It's fun and simple. I recommend this to anyone who has ever looked at their games list only to decide not to game at all.