Last week I was in NYC, we stopped to take a real coffee in a small coffee store and I see the payment system they have is a tablet with a credit card reader.
I took the coffee and asked the waiter about the application they use. It is Revel, from Revel systems.
Offline mode, data analysis, inventory capability, payment encryption …
So you can purchase a credit card reader and go with your tablet or smartphone and enable your customers to pay.