On the subject of hikes, bring a little backpack! We just had our beach bag which we didn't want to carry along trails with us so we just took a big gulp of water before we hiked and hoped for the best. I definitely would recommend bringing a little backpack for a couple snacks and a water bottle.
Keep a wind breaker in your bag. With Kauai being such a wet island it seems like it rained off and on quite a bit and would sometimes get a little chilly.
There is a Walmart really close to the Lihue airport and that is definitely the best place to stock up on sunscreen, snorkel gear or whatever else you may not have had room to pack in your bags. There are quite a few rental shops around the island but you can buy snorkel gear for super cheap and then you don't have to worry about renting and returning any gear.
Ditch the tourist guidebooks available at the airport and use yelp. We found that a lot of the places mentioned in the free pamphlets at the airport really weren't all that great. At night once we got back to our hotel we would just do a yelp search for whatever kind of food or activity we were looking to do the next day. We were very seldom disappointed.
Check out a farmer's market! You can try all sorts of local goodies. Tim was dying to have a fresh coconut so we grabbed one there along with a fresh papaya and some incredible "apple bananas."
Get to know people! We love striking up conversation with people we meet along trails, in the hot tub, wherever! We saw a lady walking out of a tiny, older looking grocery store with this amazing looking treat so we followed her lead in search of her yummy treat. We discovered the most fantastic juice bar ever. We treated ourselves to a yummy acai bowl.
And incase you are wondering, here are a few of our favorite places we visited in Kauai
Koloa- Aloha Aina Juice Bar (smoothies, fresh juice, loved the Aloha bowl!)
Koloa- Da Crack (mexican, I loved their pork burrito!)
Kapaa- Verde (tacos, burritos, this place gets busy so get there early!)
Kapaa- Al Pastor (taco truck, great fish tacos!)
Hanalei- Hanalei Coffee Roasters (breakfast, get the papaya waffle!)
Anahola- Ono Burger (burgers, I'm obsessed with their onion rings!)
Hanapepe- Kauai Kookie Factory (tons of cookies that are so different and fun. My favorite was that they let us sample anything and everything. We were stuffed by the time we left!)
Have fun! I loved that Kauai wasn't all that touristy. There is really so much more to do than to just lay around at the beach. Go explore and have fun!