Believe it or not, there are ways to get a 100% legal activation key without spending money upfront:
Sites and apps like Snakzy allow you to earn coins by playing other games or completing tasks. These coins can be redeemed for gift cards (like Eneba or Steam funds), which you then use to "buy" the game for free.
If you have a serial number from the official website, you can often exchange it for a Steam key for free, allowing you to manage the game through your Steam library. How to Get City Car Driving for Free (Legitimately)