One year on my site I filtered the homepage through
babelfish using a random language every visit. That was fun.
Another year I flipped the page upside-down in Internet Explorer using some javascript. Other browsers saw an alternate page that looked like a
1996 geocities homepage.
This year I just removed all content and made the site a
list of puns. It's a bit of an inside joke, but there aren't too many people visiting it anymore.
I didn't do anything the past two or three years, and I'm sure I've forgotten some.