I agree with the idea of prescription goggles for snorkeling, if you wear glasses and your prescription calls for more than about -4. This will make a real difference to your experience in the snorkeling reef. On different trips, we went on 2 snorkeling boat tours, one that stocked prescription goggles and one that did not--with no obvious indication up front. The prescription goggles give you a very much better experience. Although I have now purchased my own prescription goggles, I am in the market for lighter ones--I don't recall the ones I had on the boat being quite this heavy. You may also want to bring your own snorkel.
I purchased the anti-jellyfish lotion/sunscreen combination for a Carribbean trip. I am not sure whether there is any need for it at Aulani.
For people who don't need or want to carry their phones around in a waterproof case, one of those smaller waterproof containers that holds credit cards, drivers licenses, etc. is valuable.
I like the 2.5 gallon ziplock bags for bringing wet swimsuits home.
A sun hat is a good thing to pack. There is not a lot of choice around Aulani, in my experience. If you like visors or baseball caps, you can get them there, though. I also recommend comfortable flip-flops, underlining comfortable.
I think we purchased souvenir beach towels at Aulani. They take up a lot of room in a suitcase, and you can do fine with the regular-size towels that are provided.
The coconuts that they sell at Aulani, for drinking coconut water and I suppose eating the coconut filling itself, are intriguing for people who live far from coconut trees. It is sometimes hard to open them, though, because the opener tabs they put on them often break off before the "plug" (about as big around as a penny) pulls out. I would have liked to have a mini screwdriver or other tool that would accomplish that.