I don't do practical jokes much anymore -- let's just say that when I was younger, it would frequently happen that this sort of thing got escalated quickly into territory that put large swaths of the population at risk of life and limb -- and I almost never do them on the blog.
This post on Blastr yesterday reminded me of the one time I did, though. It came to me in a flash, pretty much all at once, due to the setting I was in at the time. I took the shots, set everything up, and then sprung it on my readers two years ago on this very day.
Of course, you know going in it's a joke, but I still think it's funny. And since I'm lazy, I'll simply point you to the post: Spirit sees phenomenal Martian vista.
Enjoy. And, of course, April Fools!