I grew up in Honolulu and recently got to go back home. We stayed in a house on Sunset beach just down the road from Haleiwa. I was worried about being far from town and all my favorite food, but found some good things to eat.
By far the two best things I ate on the north shore are:
#1 Poke from Kahuku Superette
A cute little general store with absolutely fantastic ahi poke. They seem to be known for their shoyu poke but the limu is great too. You can get a tub of poke on top of a scoop of hot rice and bring it the beach (Malaekahana for example). This might be the best raw fish I had the whole trip (comparable only to Ono Hawaiian in Honolulu). It was so good we went back a second time.
Kahuku Superette
56-505 Kamehameha Highway
Kahuku, HI 96731
#2 Huli Huli Chicken at the Waialua Farmer's market
We didn't make it to the Haleiwa farmer's market (sundays) but did go to the Waialua one on Saturday. Very cute market next to an old sugar mill with great fruit and produce. But the best part was a huli huli (hawaiian rotisserie) chicken stand. Get it chopped up and try to wait until its cool enough to eat with your hands. I can also recommend the smoked marlin if the fish guy is there and the shave ice from inside the coffee factory (made with fresh fruit puree instead of syrup).
We didn't try many of the sit-down restaurants in Haleiwa because I was focusing on local plate lunch and takeout type stuff (and Haleiwa seems to cater more to hippy surfer type food), but Kua Aina sandwiches is famous and has pretty good burgers and mahi mahi. And the Foodland grocery store at Sunset beach carries locally made Pork Lau Laus (pork and taro leaves steamed with salted butterfish inside a ti leaf) and Poi that will do the trick. One other note; the shrimp truck scene in Kahuku has changed a lot since I lived in HI and I didn't know where to go. We were disappointed with Giovanni's — bitter off garlic flavors, mushy rice and shrimp, no good sear, etc. You may have better luck with some of the other trucks in the shrimp fields. Or just get more poke from Kahuku superette!
And one Honolulu tip: Ono Hawaiian on Kapahulu. It's famous, but rightly so. Get the Lau lau, squid luau (taro leaves, coconut milk, octopus), lomi ahi (hand-torn tuna chunks), poke ahi (chopped up tuna), poi and rice. Also maybe the kalua pig (pulled pork). Then walk a few blocks up Kapahulu and get hot malasadas (Portuguese donuts) from Leonards.