Ethereum has the best business model thus far, with their providing a blockchain for anyone to build their project upon. You can explain to your Grandparents what blockchain technology is and if they get that concept and utility you can tell them that Ethereum is the first company through the door offering the blockchain for the world to use.
Ripple is also one that could resonate well with an older crowd if they come from a finance or business background and can understand how slow the current payment system is. The fact that the fastest way to get $10,000 from one continent to another is to get on a plane and fly it there is absurd. The current banking system sucks because no one has ever challenged it. Ripple seeks to challenge the system, but not leave them behind, Ripple wants to bring them along. It's a happy medium and there's a ton of potential.
What work background do your Grandparents have? Tailor the token story to what they know.