This pass summer I gave out a homework suggestion to some of the team members I know. (Whom did I learn that from?)
They were to keep an eye open for junk. Not just any junk but useful junk that the team can use for constructing robot prototypes.
The best find, in my opinion, are discarded wire spools. They make perfect robot wheels and comes in many sizes and materials.
The next best find are shopping carts and baby carriages (no babies were harm in this assignment).
Honorable mention was given to the aluminum tubing from old lawn chairs.