You don't need a bike rack! Find a fence post, flag pole, light post, or other fixed item to lock your bike to. Like someone else said, don't lock your bike to the same place regularly as it'll bring attention to the bike thieves. Usually fixed items in the wide open are great candidates to lock your bike to as most thieves won't want an audience while they cut your lock. Take any personal belongings with you (don't even leave a saddle bag with a spare tube/wrench set).