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Let me start by saying that I've only been to Kauai once, and it was before my interest in photography. So, I've heard/read that there are some decent landscape ops there (eg. Na Pali coast, beaches). Definitely go check out the different beaches and tide pools. Obvious ops here are landscapes, macros, and shorebirds.
As for birds, there are a lot of feral chickens around. Did I say a lot?! =D
Some Internet resources suggest that there are places/trails with some native birds such as the Hawaiian Honeycreeper (I`iwi). Definitely checkout Kilauea Point National Wildlife Refuge. Depending on your timing, you may spot various migratory shorebirds, waterfowl, Hawaiian Goose (Nene), Hawaiian Short-eared Owl (Pueo), etc. There should be some feral pigs about, too.
I agree that the 5D3 would probably be the most versatile body for you on this particular trip. Those three lenses should cover a lot. While a 16-35 would be really nice, don't discount the 24-105. You could always shoot/stitch a panorama.
I definitely need to take the family one day myself. For me, it's all about the hiking, birds, flowers, plants, and ocean.
Have a great time, and best of luck finding those hot spots.