I have done parrot shows (Yes, there is such a thing!) for 7 or 8 years. I would always do as others have mentioned, just write down the numbers and run them later at home. I had several that were declined, but eventually they all went through in the next week or so. I never did have a bad check. After having done that and currently having a successful internet business that accepts credit cards, I would offer a few comments.
1. The PayPal terminal would work on a laptop, but it is NO where near the cheapest option and might even be on the more expensive side. You also might not know if you were going to have wireless service until arriving at the show, and then your whole plan goes out the window.
2. Don't take checks period. Only take cash and credit cards. (MY opinion!)
3. The surest way to get paid is to just telephone in the credit card number to your processor. They will have an 800 number just for this and you call them and tell them that you want verbal authorization for a charge. They will authorize it right over the phone. When you get home, you run the charge (it is called "force the charge" in credit card lingo) on your machine using the information that you were given over the phone at the show. At that time the charge will actually process and you will get paid.
4. If you decide on some other way, you will need the credit card number, the CVV2 number, the amount of sale, the address that the card is billed to, and the zip code that the card is billed to in order to get the best rates. If you don't have the zip code for example, you will probably pay a bit higher rate than if you had all of the information. If you do not have the CVV2 number, the charge probably won't go through at all. In this era of scams try calling someone and tell them that you just need the CVV2 number off of the back of their credit card because you forgot to get it and see what they say.
5. I personally use the Costco platform. They do not have a monthly fee and they do not have a statement fee, so if you only do a few shows a year, you will only pay for the actual charges that you run.
Also, most of the advice in the previous replies will work just fine assuming that everything goes through. When you have any sort of problem though, you are going to come out on the short end, as you are probably breaking every credit card acceptance rule there is and I am sure that you signed a contract saying that you would abide by all of their rules.
I hope this helps...... Good luck with your sales.