Not sure. But I'm pretty sure they're reaction will be "Your givin the tools to create anything you want, why does everyone in your time seem to always make it brown, grey, on Earth, and about some type of military?"
In which case, I would show them Mass Effect 1, Fable 2, and Shadow of the Colossus.
Mass Effect because it shows how big games can be. In terms of worlds and story. Plus, you don't go on Earth. And I chose the first because I felt it had the better narrative. Plus, it shows the Galaxy. And since the time you stated is post Star Wars/Star Trek, well you have a lot of people who can finally live their dreams of seeing the galaxy.
Fable 2 because I want to show them how not to do an RPG story. But everything else in the game, the beautiful world, the dark, funny humor, the unique, fun, interesting, and funny sidequests, and how much freedom games can give you. How some things that seem big don't matter that much, and how small things can matter a lot. It also shows how hilariously broken game economies can be. Which considering the game itself I can't tell if that was intended or not.
Shadow of the Colossus because it shows how less is more. How only 16 enemies is still more fun to take down then endless waves of faceless baddies. And how amazingly fun it is to scale giants, and them bring them down. Plus the story is phenomenal, and shows how gaming can be a powerful narrative with little to no dialog.
Also, another reason I picked these is because they don't have multiplayer. Well Fable 2 does, but that was an afterthought and shows that not everything needs multiplayer. If they saw what multiplayer did to gaming as a whole, or basically what xbox live did (birth multiplayer-centric games, birth xbox live douchebags), they would cry.
