When we were in college and completely broke, a few friends in our dorm came up with the concept of hunting for FreeFun. One of our best discoveries was TrayBall -- best played on the lawn behind the UCSB UCen, in front of the lagoon. Basically TrayBall is a cross between baseball, a food fight, and skim boarding.
Step 1: Borrowing
Discreetly (ahem) "borrow" a few of the plastic trays from the dining commons -- you need about 8.