I have to confess that I'm kind of terrified of clowns and always have been.
This is not to say that I'm opposed to clown-related pranks and events, merely that the thought of them fills me with fear and Sartrean nausea and makes we want to find someplace dark to cower for a long while.
Maybe a clown prank in an environment full of people scared of clowns might work. For instance, if we could find someone as uncomfortable about them as I am and then hide clowns throughout their workplace and escape route, that could be pretty funny.
(I suppose our pranks shouldn't be designed to give people PTSD though....)
Freezing could be cool too, but I can't help but feel that it's been done already by others and that we can be more original than that. After all, we are creating a sort of art, and the best art doesn't copy its influences but uses them in new and original ways to create something different and unique. Plus, when videos of these missions go out onto the Internet, they might be dismissed as derivative if we don't make sure that they're unlike what the world has already seen before.
This is all just my opinion, of course.