There are auto-feeding devices on the market.  Most petshops/aquaric shops will carry them. 
Goldfish can go without feeding for as much as two weeks.  They will eat algae growing in the tank as well as nibble on any plants you have in the tank.  The feeder blocks are not good to put in the tank (weekend feeders).
They just end up fouling up the water.  It's better to let them go hungry rather than pollute the water.

You can easily leave the fish there, there WILL be something for the fish to eat in the tank no matter what. And I'm afraid that if you bought plants with thin and small leaves, there won't be much left of them soon My fantails totally tore my smaller plant apart... xD (And they were like 1.5 inch long at the time)

Don't give them any more than usual for a "goodbye" this will just pollute the water unnecessarily as goldfish don't have stomachs, and the food goes straight through them! They just collect the necessary nutrition, and that's it. So feed them 3-4 times a day, and in small amounts. But like mentioned earlier, they'll be fine for two weeks! (:
If it'd make you feel better, you could try and give them some freeze-dried live foods (Blood worms, tubifex, krill...) Just before you leave, it's more nutritious than normal flake foods (:

If you do go with a fish-sitter, be sure to be clear on feeding times and amounts.  Putting the food for each day in little individual bags is a good idea, as is giving your sitter a measuring spoon or some device.