Educate yourself
If your mind needs stimulation, take a course at your local university or community college. Many of these institutions allow seniors to audit classes for free, although you often do not receive academic credits for taking courses this way.
Perhaps you’ll be so excited by your new learning that you’ll decide to pursue a degree so you can “unretire” and enter a new line of work. But before you do, make sure you’ve thought the decision through carefully. Get a start on that process by reading the following: