Bones

Bones is a business card size TTRPG built to be as simple as possible. Its great for letting people who don't play TTRPGs experience the fun of them. All you need is the rules, at least 1d6,  paper, and pencils. I urge you to try running this system for your grandma.

Warning: One side effect of this system being so simple for new players, is its possible difficulty for new games masters. Many aspects of the game are left vague due to the space constraints. My advice, just run it however feels right to you.