Sbe727 raises the best bet:
Most of the larger airports in the US have ground handling companies that the cargo carriers (DHL, UPS sometimes, US Postal Service-Ryan, etc.) contract with for staff and equipment. You can be a ramp agent just like the airlines, but with mail sacks and boxes instead of luggage. I did that for three summers with a company called Worldwide Flight Services at GSO
. It was dream-come-true work for a true airport-lover, although the work day was from 9pm to 6am the next morning!
Worldwide has a big division at ORD
But when I worked for them it was before September 11th and security was no big deal--- now I'm not sure if it's as feasible for a company like that to hire someone for only seasonal work. But there's no harm in asking.