1. The batting cage idea is very risky. I’ve seen many of them close over the years and it is not anything close to passive income if you want to keep the business going. You have to continually promote it and target youth leagues, coaches, schools etc to catch all of the new players who grow up and want to play. I’ve played at probably 8 batting cages over the years and 7 of them closed.
Secondly – and this is just quibbling – I’d change that risk score. The risk of private equity is incredibly high and should be considerably riskier than bonds! You are providing a typically very large amount of capital to one business that you agree to have no control over, and the success or failure of that business over a locked, predefined term determines your return. And in the few deals I’ve negotiated for clients, my experience has been that there are often management fees, performance fees, etc. that may cut into your potential gains, anyway. You’re putting a lot of eggs in one basket, and promising an omelet or two to the management no matter what. You really need to be confident that you found the next Uber before you take this giant risk!

Stocks, bonds, 401(k)s, annuities, etc. are great ways to earn passive income. If you're not financially savvy, you'll want to hire a financial advisor who'll help you choose the right investments for you. You may want to take a couple of finance classes to understand what your advisor is speaking about when he/she recommends an investment strategy. Done right, investments can pay off for years.
Investing in coins and collectibles: Buffalo nickels and Spiderman comic books are good examples of coins and collectibles that can rise in value, and thus offer opportunity for passive income investors. You'll need to get up to speed on the value of any coin or collectible under consideration, but once you do so, you're on the way to price appreciation on a commodity you'll be paying a lower price to buy, and will garner a higher price when you sell.
If you have specialized knowledge in a certain topic, you can put together an online course to teach others. For example, if you have experience in real estate investing, you can create an online course “Real Estate Investing 101”. The benefit of an online course is that once you create the course material, you can sell it to as many people as you want.