I wanted to say the same thing. Start a small business on the side. Something you enjoy; there's a million ideas. A friend of mine buys and sells classic motorcycles. Someone builds new replacement parts for unsupported industrial equipment. Car accessories, anything.

It should not have the potential of losing a lot of money, because you don't want to worry about it and you have a full-time job already. So that rules out big capital investments, machinery, etc. You will need to sacrifice margin (so you'll learn what that means for sure). You will need to choose your market carefully and learn a lot about it.

You don't need to make a big profit, huge revenues; you're in it for the experience. Go low overhead: work out of your home, store things in your garage, don't mess with business stationary until you need to, etc. After a year, you'll be learning about financial statements (but you should still study those of other companies as others here have said.

Make sure to keep the business finances separate, too.