Aloha H. :)
We just got back from HI with our 17 month old. If you can, bring the stroller to the gate so you can put either the kid or carryons in it on the way to the gate. If there's time, find a little spot near the gate for her to run around a bit before boarding to get some of that energy burned off. For the flight, bring lots of little games/activities and snacks. When all that gets old, flirt with the flight attendants in the back of the plane :) If you're lucky, she'll nap! I have found that traveling with little ones brings out the best in everyone around you -- from the parking shuttle driver, to the security personnel, everyone is just really kind and accomodating.
As for packing, lightweight cotton is the way to go. It's cool but can help protect from the sun. We actually love the the lightweight cotton long sleeve button up shirts that seem to be everywhere right now (old navy - check the boys section too) because they're easy to put on over bathings suits etc and are cool and protect from the sun. You'll (I'm guessing) have access to laundry which is always a big help!
Apply sunscreen before you leave the house/get to the beach (recommended for best protection and less sandy). Get a good, safe brand like Badger. Hats are a must (if she'll wear it ;). If you can hit the beach either early or late in the day to avoid the super strong heat/sun rays.
Sometimes the best way to nap them on vacation is in the car... there are so many great island drives to take!
Buy diapers when you get there.
I think staying with friends is going to make things a lot easier - check with them, since they may have a lot of things that you might not need to bring! It is a lot of work, but it will be fun. Even if you get to the beach and all they want to do is play in two square feet of sand all day, it'll be great. Enjoy and let me know if you have any more questions! (My husband's family is from Kauai and I've been going there since I was kid.)