Interest-earning accounts include several products available from banks and credit unions such as savings accounts and certificates of deposit (CD's) as well as US government savings bonds. These all guarantee to keep your principal intact (although perhaps not very easily accessible by you) as well as pay you interest. However, because of the effects of inflation they are not guaranteed to maintain your buying power even though TIPS (Treasury Inflation-Protected Securities) do make an effort to try.
Stocks and bonds (both corporate and municipal) purchased either directly or within a brokerage or retirement account offer a variety of risk/reward opportunities. It is possible to lose your entire investment with these products or make a great deal of money with them.
A third option is direct or peer-to-peer lending. In this scenario, you can find on the internet various websites that allow you to select different individuals or projects to whom you would like to loan money. As they repay the loan with interest, you receive monthly income. You can make one or several large or small loan(s), and you can underwrite a loan all by yourself or fund just a portion of a loan (meaning that other people would have to fund the rest). In this way you can create a portfolio of loans that makes you just like a banker.
Of course, if your mortgage is paid off, perhaps you would like to consider buying an investment property for resale or for monthly rental income. If you don't want to be a landlord, you can also arrange for a property manager to handle all of the details for you. I would be glad to consult with you about all of your real estate questions and interests!