@littlek,
I recommend something excessively adventurous, certainly memorable to all in your immediate sphere of influence, and not too painful either emotionally or physically -- which could mean some combination of alcohol and public nudity, but without any unfortunate photographic evidence.
Confiscate cellphones in advance of the event.
Then again, maybe you could just bungee-jump or parasail (sober and clothed, I'm meaning). Have you ever parachuted? That's a good one for 40 ...