Share Your Knowledge
Even if teaching wasn't your profession, it's still something that you can take up after you retire. Not necessarily teaching children (although if you're interested in a second career, it's a possibility), but teaching other adults what you've learned in the years spent in your chosen industry. Some colleges and technical schools like to employ people who have a lot of real-world experience, even if they don't have teaching experience. Many companies also employ career coaches or bring in speakers to share their knowledge with their employees.
This could be a paying gig, or you could do it simply for the joy of helping others and giving back to your community and other people in your previous profession. You could work with an organization such as SCORE, the Service Corps of Retired Executives. SCORE bills itself as "counselors to America's small business." Volunteers with the organization mentor small business owners, provide counseling, create and lead workshops, and write articles.
Also consider teaching other skills you've acquired that don't necessarily have anything to do with your career. Local community schools hold classes in everything from foreign languages to basics in horseback riding. If you've long been quilting, growing a vegetable garden or crafting wooden children's toys, you'll get to teach people who are interested in learning.