While traveling either West or East bound on I-70 take the rest area exit and take a break from your travels and while you're at it, CHECK YOUR MAIL.
Be aware - You don't have to drive into the rest area to get this one ! ! Watch out for Wile E. Coyote, the Road Runner, and that Raskaly Rabbit ! ! 
Ha Ha ! ! Good Luck and happy hunting, for the cache that is ! ! 