A quick and easy box-kite with only a single spar. Most of the materials you need can be recycled from your local supermarket or burger joint. My local supermarket sells fresh pies in flat-bottomed bags, takeaway bags also work well, but this version uses two of the brown grocery bags given out by American supermarkets.
Note: I originally wrote this Instructable in 2007, and only used black-and-white diagrams. I decided to update it in 2012, when in San Francisco, and all our shopping came in paper bags instead of plastic carriers. For the sake of history, I have attached a PDF of the original version, but don't feel obliged to read it.
Step 1: You will need...
Two clean, flat-bottomed grocery bags, quarter-inch dowel, sewing thread, and glue. The only tool I used was my Leatherman multitool.
You can substitute your own materials wherever you like (for instance, you could use the bag your burger came in, and the straw from your drink instead of the grocery sack and dowel).
(Irony: I used sewing thread to fly this version, but as we walked on from our test-flight, we found an abandoned reel of kite-line.)
Step 2: Make the sail.
Open the bag out, carefully, into a sort of box shape.
Cut round the two short sides of the base, and one long side.
Lay the dowel inside the last, uncut, long side, sticking out at both ends.
Trim the dowel so that about an inch is sticking out at each side (remember symmetry).
Fold the base of the bag into the bag (over the dowel) and stick it down, trapping the dowel firmly in the crease.