Don't throw away pizza boxes, make them into airplanes.
What you need:
1+ pizza box/boxes (with as few grease stains as possible)
1 keychain ring (circuler metal thingy)
Scissors (preferably sharp and big)
Tape (duct, scotch, blue, anything)
Ruler (trust me you'll need one)
String (sorry it wasn't here before)
(I beg you to look at the comments)
Step 1: Body
Cut out a piece of cardboard roughly this shape.
From one side to the other it measures about 14"
This is your body,
next is the wings
(don't pay attention to the slits :] )