Infocom Text Adventures Online
You're in a winding N/W passage. The walls are caked with mud.
A small key lies on the floor.
What would you like to do?
If you've ever played a text adventure, you'll recognize this format right away. These days, it's known as Interactive Fiction, but that's pretty much the same thing. The game is text only (so are books, remember?) and the premises are different: sci fi, medieval adventurer, crime-solving gumshoe, archaeologist...there are different games for different tastes. The computer describes your surroundings and tell it what to do. There's usually an objective you need to complete, objects you can pick up (access them by typing INV for inventory), Non Player Characters (NPCs) to interact with and puzzles to solve. Some of these games require some pretty creative thinking in order to negotiate a particular obstacle and the best ones test your ability to think logically, to the absolute max.
And now you can play the best ones (by a company called Infocom) for free! Just go here and play away. Trust me, it's how I wasted my youth and you can't argue with the way I turned out.