Everyone seems to love "retro" 8-bit video games, but it doesn't get much more retro than a text-based adventure. If you've never tried one before, or you're just bored and have a Mac nearby, open up Terminal and give its built in MUD (multi-user dungeon) a spin.
You won't need to download anything here. Just type the following into Terminal, and the game will launch automatically.
emacs -batch -l dunnet
If you've never played a MUD before, you'll find that it's actually pretty intuitive. For example, you start the game standing on a road with a shovel nearby. Type "take shovel" to add the shovel to your inventory, and "go east" to walk down the road. Typing "Inventory" will show you what you're carrying, "save" and "restore" let you play through in multiple sessions, and "help" gives you some tips if you're stuck. I'd also recommend using some scratch paper to map out the area as you discover it. If text isn't your thing, you can also check out some hidden arcade games via Terminal.
Terminal 101: 4 emacs Easter eggs [Mac|Life]