Day Spa Disaster

Construct "Day Spa"

This will need to be a temporary building that is free standing and preferably a rectangle (for maximized destruction!) with the entrance leading to two rooms in the back, like so:

There should also be some sort of pool or a body of water nearby.

Lay the Trap

The first room is reception/welcome where the mark is greeted by the staff.  The second room is a massage or something to relax the mark and take their mind away from the fact they're about to be taken for a ride through hell.  The third room is where it all comes together.  Upon entering, the mark will be blindfolded, preferably with something that will be EXTREMELY difficult to remove (it enhances the effect!).  Substitute the bucket of a front end loader for a tub and fill it with mud.  The mark will enter the mud bath and begin relaxing. 

Bring on the Prank

Approximately 5-10 minutes into it, someone fires up the engine of the front end loader.  At that point, the mark may suspect something but will be unable to remove the blindfold and will be understandably panicky.  From there, whoever is driving the front end loader raises the bucket and tears ass through the building smashing it all to hell.  Drive around for a few minutes to guarantee several looks of terror and follow it all up by dumping the mark and the mud into a nearby pool.

Conclusion

Fish the poor bastard out of the water and enjoy!