postcards cafe in hanalei is great, quiet, more romantic (http://www.postcardscafe.com/). it requires reservations but is casual, all organic. dolphin cafe has fantastic grilled and roasted fish dishes, doesn't take rezzies, but has a nice outdoor area to chill for drinks while you wait. sushi blues, in ching young village, has a cool vibe -- sit at the sushi bar and the guys will make some special rolls for you.
at the princeville hotel, in princeville, there's a great sushi night on monday and friday nights, friday night have live music. i like java kai in hanalei for breakfast and smoothies -- i think there's a java kai in kapaa also, and lotus root cafe in kapaa is a nice place for morning tea. ono burger, which is a roadside stand near kilauea, has delicious fish sandwiches and burgers, and the smoothies at the guava plantation are hard to beat. in kapaa, kintaro is also good casual sushi, along with some other korean/japanese dishes ... sensing a theme here?
and if you like raw fish ... ishihara market in waimea has the best poke on the island! i like the kimchi tuna, but it's possible that's just the korean in me :) aloha!
I was just in kauai last week and would recommend Coconuts for dinner. Great atmosphere esp if you can sit outside. Ahi was excellent as was a chicken dish a fish hater dined on. For breakfast you must go to Eggberts in the Coconut marketplace and order banana pancakes (or another fruit) and try the coconut syrup with em. Completely worth it indulgence. Blooming Lotus for vegetarian also very good. Tropical Burgers in the marketplace had good burgers with a twist, the guava bbq sauce burger was tasty! All right in Kapaa. Don't miss Olympic cafe for happy hour upstairs overlooking the street. enjoy!
If you are looking for fine dining reviews, primarily in the Poipu (just S of Kapaa), you might want to look at: http://www.chowhound.com/topics/334698
We only got to Kapaa for some gallery hopping, so I cannot comment on the dining scene up there. Since A Pacific Cafe closed, I'm not sure what's in/around Kapaa now.