We have been to Hawaii several times and stayed only at one island per trip. Our first trip was to Maui, then Oahu, Kauai, and the Big Island. We only stayed on Oahu one week which I felt was sufficient. We did Pearl Harbor, Hanauma Bay for snorkeling, Polynesian Cultural Center, North Shore and the shrimp trucks, hiked Diamond Head, etc. We spent at least 10 days on Maui and the Big Island. Of all the islands, Kauai remains my favorite because of the Na Pali Coast, Polihale Beach, and just that laid back Hawaiian feeling. Best luau though was on Maui.
We are going to Aulani in November and most likely will stay at the resort the majority of the time to enjoy what it has to offers. We'll probably go to
Waikiki to stroll and see how the waves are on the North Shore.
All the islands are beautiful and if you can only go for a once in a lifetime trip then don't spend all your time at Aulani. In fact, Aulani is on a side of the island that I don't particularly like other than the Ko Olina resort area.
If you really want a taste of the real Hawaii take the road to Hana on
Maui, watch the sun rise at Haleakala, do the Trilogy snorkel trip from Maui to Lani, do a plane or helicopter tour of Maui which goes to the Big Island so you can see an erupting volcano, take a boat ride along the Na Pali Coast, etc. etc. Also see my favorite beaches Makena ( Big Beach on Maui) and Polihale Beach on Kauai. Rent a car and see as much the islands as your time and budget allows.