Each of those choices are very different. Each island is also very different. You'll want to ask how your nephew wants to spend his time, and how much time he will be spending in the resort. if he's like most younger people, he'll want to be outside, enjoying the Hawaiian sunshine.
Oahu is an island, and NOT just a big city. Yes, Honolulu is there, and Waikiki is part of Honolulu, but that's only part of the island's appeal. Pearl Harbor and Diamond Head are on Oahu. So is Waimea Bay, Makaha, Pupukea, Pipeline, Haleiwa, and Sunset Beach, for anyone who ever thought about climbing on a surfboard. A drive to the North Shore beaches brings a totally different experience than being in the city. Hiking the Koolau Trail is an adventurous, exciting time. There is plenty to do on Oahu, in or out of the city, especially if he has a friend or two along to share in the fun. If the goal is to escape the winter cold, then just kicking back on the sand in Waikiki may be a fine time, but a day spent in the sun on Kailua Beach may make him a Hawaii fan for life -- it's one of the best beaches on any of the Hawaiian islands. I used to live in Kailua as a teenager, and every time I go to Oahu, I make it a point to spend time on Kailua beach. In my opinion, none of the other beaches in Hawaii are as nice. Waikiki nightlife is as wild as it gets. No limit to the fun, people watching, and entertainment options to be found there. Definitely the "South Beach" of Hawaii.
Kauai is a quieter place. Great hiking, great beaches, and fewer people. But the Ka Eo Kai resort is perched on a cliff high above the water, and if you want to do anything, you need to have a car to get there. For anyone who wants to get up and out and "do" stuff, Kauai may be a bit too laid back. There is virtually no nightlife on Kauai, so if he's interested in after-dark fun, there isn't much on Kauai.
The Big Island is a bit of everything. It's very big, so lots of driving is required if he wants to see the high points. But if he wants to see an active volcano, the Big Island is the only place to see one. Black sand beaches, the telescopes at the top of Mauna Kea, amazing hiking in the lava fields or through the rain forests - there is plenty to do there. Snorkeling is excellent, and there is plenty of Hawaiian history there. It's a place for a more adventurous type.
Whichever you choose, you can't go wrong. They're all beautiful, exciting, and fun. Just very different from each other. The resort is secondary to it all.
Dave
