While you may like to have a fine dining experience or two while in Oahu, for the most part you can find food that meets two essential requirements:  cheap and tasty.  There are plenty of places to eat in Waikiki, but a favorite is the International Marketplace.  The Marketplace can be quite a labyrinth, so the easiest way to find the food court is to enter the Market and look down:  somewhere nearby, you are bound to see a yellow stripe with arrows, so follow that (if it helps, remember:  "Follow the yellow brick road.")

Just as the name implies, the International Marketplace offers plenty of international cuisine.  Everyone serves big portions, so you can buy one meal at one of the booths and a fruit salad from a booth near the alley, and share that.  You can also share a large drink or a bottle of water that you can carry with you everywhere you go, although there is an ice cream stand at the Marketplace that sells excellent fresh pineapple juice.

A great place for breakfast is the Waikiki Beach concession stand.  You can get a choice of meat (bacon, Spam, etc.), eggs, and rice for around $5.  If you're on the beach all day, it's a good place to go for lunch as well.  If you want good coffee, just go around the corner to the Honolulu Coffee Company at the Sheraton Hotel.

Kailua Beach also has a concession stand, and they serve a really good teriyaki burger and fries.  You can park in one of Kailua's lots in order to walk over to Lanikai Beach (go up the hill, then take one of the public access walkways toward the beach), and half the time you'll pass a big, white concession truck.  The lady there also sells good, cheap, tasty food, although they sometimes run out of the special of the day pretty early.