I am vegetarian and my daughter's diet is gluten-free.
Most breakfast and dinners we ate at condo, packed lunch to go on some days and ate out lunch or dinner on other days. We brought a pack of gluten-free cereal and a pack of snacks from home which helped during rainy weather and tropical storm on the first few days of out trip.
Restaurants (listed in order starting from what we liked the most to what we liked the least):
Dinner at Hukilau Lanai at Kauai Coast Beach Resort in Kapaa was great. They had separate gluten-free menu and gave a lot of attention to make sure all items are completely gluten-free. My daughter especially loved their homemade gluten-free bread. We didn't make a reservation so were sitting in the lounge closer to the musician rather then dining room but the view from were we sat was actually better then from the dining room. Beautiful life music, beautiful view of garden and ocean, delicious food, excellent service. We both loved it and will come again next time.
Delicious lunch at Mediterranean Gourmet overlooking the ocean. Loved everything-food, service, view, proximity to Tunnels and Kee and most importantly that they had many gluten-free and vegeterian items clearly marked as such on the menu. Will come again when visit Kauai.
Dukes barefoot bar downstairs in Lihue. Loved the view, food, atmosphere and service. Puts you into island mood right away. Awesome salad bar for me ($17 buffett), good chicken for my daughter.
Dinner at Oasis On the Beach overlooking the ocean- liked the food, service, and especially the setting and the view. Watched a sunset there while having dinner- beautiful. They have all items marked on the menu if they have gluten and dairy or not.
Lunch at Kalapaki Grill in Lihue Marriott (veggie quesadilla with extra avocado for me and fries for my daughter) was very good.
Lunch at Hanalei Gourmet- the food was just ok, very few gluten-free options but liked the place, location and cheerful atmosphere. Service was good. Chicken was not gluten-free so my daughter had a burger without bun but didn't like it. I had Waldorf salad. They had several salads so easier to find vegetarian options there but very few gluten-free.
Olympic Cafe in Kapa'a upstairs. Huge portions but so so taste. Nice view. I had monster burrito, my daughter had chicken, both very spicy. Many people seems to like the place and it was busy but we didn't really like it due to the very limited number of gluten-free options and rushed service.
Other places that possibly have gluten free food that we read about but didn't visit yet. The info below is from multiple posts of other travelers:
Sweet Maries in Lihue close to the airport. They are 100% gluten free.
Cafe Coco in Wailua
Mermaids in Kapaa
Dolphin in the Kukuiula Village has a whole separate gluten free menu.
Living Foods market in Kukuiula shopping center has GF granola.
Tortilla Republic in Kukuiula near Poipu had many GF options on their menu
Merrimans and Beach House have GF meals. Merrimans downstairs had the best gluten free pizza.
The Garden Cafe at Common Ground (vegeterian, gluten-free)
4900 Kuawa Road, Kilauea, Kauai, HI 96754
Kilauea Bakery always has gluten-free goodies
Postcards in Hanalei.
They note which of the menu items is GF.
We also ate shave ice everywhere we seen it and liked Hee Fat in Kapaa the most, and the shave ice from a little place on Ha'ena Beach the least. Wishing Well in Hanalei was always closed.