Another subtle thing to do is to type an "invisible" spoiler at the end by using white as the font color to give the impression of an incognito post, and possibly add a Muppicon of choice as a subtle hint.
For instance, when April 1st came along, I celebrated it in the "I think I'm going crazy" thread (which is sort of like a year-round April Fool's party, even in July, October, December, etc.) by inserting two separate yet slightly similar Muppet photos side-by-side, and I left a subtle hint like this:
:
APRIL FOOL'S! (P.S., if you can read this, you just peeked at the invisible spoiler!)
The invisible text can be highlighted by dragging the mouse over the space, which only shows when quoted and/or the blue text highlighter is used....