If you keep your money hidden in the attic or earning close to nothing in a savings account, you might want to consider putting it to better use through smarter investing as your side business idea. Sure, there are plenty dangers to look out for, but most personal finance experts will advise you to start learning about the stock market if you want to grow your wealth, and to avoid jumping too heavily into trends like Bitcoin and other cryptocurrencies without proper understanding.
I surprisingly get a lot of people asking to detail their car. I never intend to make a business of it, but I love doing it to my cars and people ask me to do it to theirs. All it takes is a cheap orbital buffer (mines a used craftsman) and a shop vac. I normally get easily $100 for a basic wash/wax/vac, or $200 to remove scratches and polish then wax the car.