When I used to do art & craft shows, I was not set up for credit cards either. It was more of an expense especially if sales are slow. Depending on the event and the people, accepting cash and checks should be fine. Do be careful on accepting personal checks, for which I'm sure you know.
I did know an artist who told me he took his laptop with him to shows. If the building or area had wireless, he could accept payments via PayPal. He would log in and send an email for payment on PayPal to the customer. He would then log off and allow the customer to log in and make the credit card payment then and there. This may be an option for you. If this works, you may also want to sign up for Payments on Amazon.com. It works the same as PayPal. If your customer has an Amazon account, you can accept their payments online as well. Though I have never tried this, I am curious to know how well it works.
Good luck at this event!
Pat