Sonic and All-Stars Racing Transformed Global Steam

I've easily racked up 100+ hours in this game and I still haven't completed it 100%. As one of the few 'less-realistic; more fun' racing games on PC, it does an incredible job. You can go through the World Tour as co-op and it is just simple fun racing with the additional element of transforming into a plane or boat to really test your abilities. It does get progressively harder and there are a number of different race styles, from simple laps, to sprints, to taking out a Boss, to drifts. On top of all that, there are a whole bunch of different characters, all with their own stats, so you'll likely find your favourite racer and go through the game with them (though if you're really good, you might just go for levelling them all up).