I’m from Kauai, go back there to visit family regularly, and have to concur with @js76wisco that you shouldn’t get your hopes up, food-wise. I usually eat at home when I’m back there!
That said,
Hamura Saimin is a real Kauai institution that I can recommend if you’re looking for local flavor.
Kintaro’s is a local’s favorite Japanese place, but it’s more popular for it’s longevity and because it’s known as a local’s kind of place (they hook regulars up), and not likely to impress someone coming from anywhere with great Japanese food.
In lieu of Lappert’s, which for me is mediocre and overpriced, I’d recommend getting a white pineapple frostie from the smoothie place that replaced Banana Joe’s in Kilauea.
As for general tips: South shore has less chance of rain, beaches that are good for lounging. North shore has more scenic beauty, more rustic, and more dramatic beaches.