Visiting /enter_spoopmode.html will choose a theme and mangle the text for any page you next go to accordingly (think search&replace with swear words or santa clause)
It will also show a banner letting the user know they are in spoop mode, with a javascript button to exit the mode, where the AJAX request URL is ofuscated (think base64) The banner is at the bottom of the html document (not nesisarly the screen itself) and/or inside unusual/normally ignored tags. <script type=“spoop/text” style='display:block">you are in spoop mode</script>
Or have a secret second page that is only followed if you ignore robots.txt /spoop_post/yvlhcigcigc is a clone of /post/yvlhcigcigc in ‘spoop mode’