The usual options:
Leave it blank.
Redirect to your main website.
Post a little message that nobody will see.
Other options of increasing nastiness:
Redirect to Google or some other benign website.
Embed
this video.
Redirect in an infinite loop.
Redirect to some three-letter agency to scare the visitor.
Redirect to
all three-letter agencies at the same time, using a large number of popups.
Link to random websites of questionable legality, causing the visitor to get busted by one of the aforementioned three-letter agencies.
After all, only bad guys type IP addresses into their browsers, right?
