1.) DIY If you love food, and have a kitchen, build your itinerary around the farmer's market schedule for the island, and you won't be sorry! (personal faves: Asian greens, little bitty chilies with a kick, freshly ground turmeric, young ginger, and Kauai honey). Then source some local fish (we looked at grocery stores, supermarkets, and fish markets, as well as farmer's markets).
Here are our favorite spots, working from west side and going counterclockwise to the North Shore:
2.) If you are in Kokee State Park stop at the scenic overlook that has a concession run by Hawaiians and treat yourself to the offerings, including an unbelievable cookie called a Pineapple Cracker. We call it "Cookie State Park" Tell 'em the Alaskans sent you! Great gfits for friends, too!
3.) Island Taco in Waimea (da kine wasabi ahi and kahlua pork items)
4 .) Kauai Kookie in Hanapepe (not a restaurant, but worth the stop for homebaked cookies for gifts or yourself: we are partial to the chocolate dipped macaroon)
5.) Mariachi's Authentic Mexican Food in Lihue -- we had lunch and breakfast on the lanai, and could see Nawiliwili Harbor while enjoying great service and great food. You can even get nopalitos (cactus) or chilaqhiles with your breakfast eggs. Muy sabroso! We are Mexican food snobs and we were happy. Easy trip to the airport from here, too.
6.) Papaya's Natural Food Store in Kapaa - This is a health food store with broad range of offerings: at lunch time they had soup, salads, sandwiches, hot dishes, Island specialties: there are a few tables outdoors and you can eat on the premises or get items for a picnic. It is set back from the highway in a shopping center.
7.) Postcards - Hanalei: fine dining. Yum.
8.) Neide's - Hanalei. Casual indoor or outdoor seating: We return for the mellow vibe, not only chef owned and operated: the chef owner of the eponymous place was cooking up the food the night we were there. They have Mexican but we always get the Brazilian food and have cocktails.
9.) Tahiti Nui - The food and beverage are just fine, but people come for the social atmosphere, created by the live music. Enjoy island musicians and hospitality: a tradition carrying on now from one generation to the next.
10.) The mobile purple food concession, sorry I don't know the name, we found it parked at Anini Beach: the gourmet salad and dressing were a surprise: I wish I had been able to try everything on the menu! Ono grinds served up by these folks.