Here's a text-based real-time "survival horror" game with surprisingly strong replay value, for such a passive act. Its title is its goal: Don't Shit Your Pants.

You'll figure out how not to defecate in your britches easily enough, but once you get a peek at the 10 achievements for the game, you'll start experimenting with all the ways you can send the brown fax, whether that's in the bowl or in your drawers. I've got about four of the achievements so far. Let me know how to get the others, in the comments.

Don't Shit Your Pants [Kongregate, thanks Teddy L.]