My husband, 20 yr old daughter and I are flying to Hawaii in August to visit my 24 year old son. We are thinking of splitting our time between Oahu (where my son lives) and Kaui. Do you have any advice on what to see and what to avoid?
Do you like to hike? Swim? Snorkel? Shop? Skydive? 
We hike. We have never snorkeled. We love good fresh food. I am an avid long distance runner.
I strongly recommend using Tripadvisor’s Hawaii Travel forum. We’re taking a trip to the Big Island in September and I have found the forum invaluable!
Kauai is beautiful. One thing we did that was out of the ordinary (at least for us) was to take a helicopter tour of Kauai – it was expensive but the views were amazing. We also snorkeled, enjoyed the beach etc.
Helo tour of Kauai was worth the price. Lots of good hikes. For Oahu, visit the North shore. Drive East/West God vies as you cross the ridge. Ford Island and USS ARIZONA. Oahu for history and Kauai for nature.
The highlight of one of our trips was an 18 mile kayaking trip around the Napali coast of Kauai. It was expensive, but totally worth it. It was also not as difficult as it sounds. The “blue books” (Kauai Revealed/The Ultimate Kauai Guidebook is one of them) are also great resources. Waimea canyon is great for hiking. The Dolphin Restaurant on the North side was very good.
If you like hiking, you will be in heaven in Kauai. Waimea canyon is beautiful. Take a pair of binoculars if you can.
We also hiked a good portion of the Kalalau Trail. Camping is out of the question for my family so we didn’t go all the way to the beach.
The internet is wonderful for accessing trails before you go. My very active friends wanted to do the cave hike in Kauai. I sent them the YouTube video of the sections where you wade through waist-deep water. No way did they want to risk Leptospirosis.
We hiked the Kalalau trail and camped. It was worth lugging our tent with us! 
You will need to pay for and print a permit off the HI parks site if you want to hike past a certain point of the trail. Ditto camping.
There is good hiking on Oahu, too.
http://www.honolulumagazine.com/Honolulu-Magazine/September-2013/20-Great-Oahu-Hikes/
If you have not snorkeled but can swim, learning to breathe through a snorkel tube is easy. Try Hanauma Bay - there is usually lots of fish, and the waters are calm and relatively easy for novices.
Any advice about luau’s and/or restaurants?
Kauai Pasta Lihue. Yum. They use fresh local ingredients.
My family really enjoyed the Chief’s luau at Sea Life Park on Oahu. We drove, but there is bus transportation available from several hotels in Waikiki Beach area. It was quite windy in January, take a jacket!
Agree Hanauma Bay is a good spot to try snorkeling. (Go early!)
At Poipu beach on the south coast of Kauai you don’t even need a mask and snorkel, goggles are fine. The fish are all around.
Shark’s Cove on the North Shore of Oahu is wonderful, but a bit shallow considering the spiky things below. You are in a protected old caldera while waves from the ocean crash around the outer perimeter.
We didn’t hike down into Waimea canyon, it gets very hot when you go down (and this was February). We hiked a bit in the state park near there and I regret not doing the hike to the center of the island where it is always raining.
We hiked a bit up from Ke’e beach on the Northern edge of the Napali coast of Kauai. Really spectacular views from there but a somewhat busy and crowded hike.
We tried to snorkel at Hanauma Bay in Oahu but it was closed. Jellyfish invade once a month, I think during the new moon so check in advance.
Kailua beach is a nice spot for snorkeling too. My BIL almost had a turtle touch him there!
Bookmarking.