Your parents sound like pretty great people. Fear of investing aside, I'd say they're more prepared for retirement than most people: paid off housing, paid off cars, some passive rental income, etc. You're also a great son for thinking ahead about how you can help them.
Unfortunately it's going to be hard to change their minds about investing. They've lived all their lives in fear of the stock market and, while they may value your opinions, they may find it difficult to accept financial advice from their son. After all, they've been managing money since before you were born.
Seconding SunshineGirl: encouraging them to keep healthy is a great idea because it's hard to argue about the benefits, such as reduced medical expenses and healthier bodies to enjoy their eventual retirement. It won't be hard to introduce changes in this area, whereas you're facing an uphill battle if you keep talking about investing.
If you still want to try getting them to invest, and they're the reading type, maybe gift them some good Mustachian books one of these days?