In Forza Street (which is a somewhat retooled version of a game called Miami Street), you can take part in races that last less than a minute or delve into a longer campaign. You’ll collect cars as you progress, and there are Xbox Live achievements.
Microsoft’s Turn 10 Studios designed Forza Street with simplified, timing-based controls to make it easy to hop into a race wherever you are. You’ll need to hold down a button (or a finger on your phone’s screen, it would seem) to accelerate and release it to brake. You can tap a second button to activate boosts once you’ve earned them. It’s not the most complex control system in the world, but at least it’s one that should make it a cinch to get through a few races while you’re taking public transit, for instance.