Some ideas for hiding Promo Codes

[Kyttias]

I've taken a tiny break from Mysidia to take a stroll around the proverbial web design block, and I've brought back with me a few fun things that are more-or-less painless to implement:

1. Konami Code - Many websites (here's a list of 20 well-known examples of sites that have) have implemented the Konami code to do some interesting stuff to amuse visitors.
2. Hidden in the Context (right-click) Menu - The best example would be to right click on the actual download link. You could probably make a really annoying game of this.
3. Sliding Door Effect - Imagine hiding one of these unsuspectingly at the bottom corner of a page!
4. On Pageview Count - This is entirely client-side, so as a warning, the same user could wind up doing it again after a cleared cache or on a different computer.

What are some ways you wish you could implement a promo code reward to appear but just aren't sure how it would be done?