The Hyatt Kauai is nice, but I'm not sure I'd call it the nicest as several others there would give it a good run. In any event, it's still Hawaii. We went to a manager's cocktail party and the manager had on an aloha shirt and slacks. Do you plan to be eating at the hotel and hanging out there a lot? If so, most of the time you'd likely be in your swimsuit. When I'm in Kauai I spend a lot of time hiking and kayaking, so those that see me coming and going certainly don't see me too dressed up. I wouldn't stress about it. There are restaurants around that are very casual. One comfortable, rather casual, attractive dress should carry through a dressier dinner.