Ha ha, reminds me of my high school days. Never used balloons though. Some things to think about would be the classic saran wrapping the whole vehicle. Or if you can somehow get the doors open, filling the vehicle up with loose newspapers is always funny. Jack the drive wheels up and place some blocks under the vehicle, until the wheels are about 1/2" off the ground, then watch them freak out when they can't go anywhere. Big zip ties around the dive shaft are good too. They slap on the bottom of the cab and make a ton of noise inside. The best One I ever did though, was wiring a car alarm speaker to the ignition fuse, so that as long as the key was on the alarm would be blaring.
2003 Ranger FX4 level II. 3" BL, 33x12.50 GT Radial Mud Claws, Custom front and rear bumpers with winch hookups, custom flatbed, with hi-lift, and full size spare mount, 12,000 lb winch mounted under bed, Electric fan, and of course a flowmaster.