SPREAD THE WORD!
livejournal, blogger, twitter, tumblr, WHATEVER. Talk about the game!
Facebook works if it's publicly visible on your wall but it's getting hard to link to facebook posts these days. Forums are okay if you're allowed to post this sort of thing there (don't annoy people by spamming, pls) and it's a forum that doesn't already have official topics. Posting about the game on the game's own forum doesn't count as spreading the word.

To qualify, you must:
1 - Name the game and link to the game's webpage
2 - Describe a mystery you encoutered in the demo
You are allowed to say that you are posting just to get a discount. That's totally fine.
For that matter, making up a mystery that you wish had been in the demo and talking about that is fine, although I'd prefer you made it clear that it wasn't actual game content.

Really, I'm not picky. Post something, then come here and post a link to whatever you posted so that I can see it, and I'll PM you a discount code.