Involves gate crashing a big awards event or film opening (like what AAF did with their song 'These Days': http://www.youtube.com/watch?v=JNM2tV3PA_8).
Pretty much cause a huge disturbance to the proceedings of an event and see how long it takes for the police to shut you down.
Another idea would be to gate crash a horse racing event while a race is in progress. Motorbikes, stunts, jumps, even skating in the premesis on the stands, running from security and see how long it takes for the police to show up. Maybe try riding a bull on the track while a race is in progress, or get a monster truck 'disguised' as a horse and once the race starts drive it onto the track as if you were a horse yourself, tearing up the grass and causing a scene that would have everyone saying 'WAT THE FUK'.
Last idea is to go dynamite surfing in a very public place.
Get some sticks of dynamite, a surfboard and a boat.
Throw the dynamite in the lake or pond, when it goes off it will form a wave.
Surf that wave in to shore.
Go para-gliding off the back of a pick up (we call them utes) down a main road.
Go paintballing at a planet-X games event. Shoot at the pros while they're skateboarding and BMXing.
Well that's all I can come up with for now. Just a bit of fun, I'm not sure what kind of stunts you are looking to pull but anyway feel free to expand on my ideas.