If your cat eats dry food... Put down a big bowl of food, big bowl of water, and an extra large litter box. We leave ours frequently for 7 days with no problem. She is of course excited to see us when we get home but there is always food left and water in the bowl. They are very good at taking care of themselves.
When we do go longer, I simply take her to the vet for boarding. Ours is a cat only vet which makes it better. One thing I have learned is boarding an animal is no better than sending your kid to day care. More often then not ours comes home with the sniffles. Which is why we opt to leave her home.