Watching my 10 and 6 year old get enchanted by Super Smash Bros for the Wii U has been really fun. I never even played it, but watching kids explore and unravel a new game is somehow way more exciting than watching an oldster like one of us do it. .
My nieces and nephew are entranced by Atari 2600 and ColecoVision games. Something magical about simple, well designed games that you don't need to read a 40 page manual to play.