There is a Target right next to Costco, about 5-10 minutes from the resort.
The Costco and Target are GREAT for souvenirs and food! We found Costco to have the best prices on stuff to bring home for our families, cheaper than the PX minimart at the military resort we stayed at.
A reasonably priced luau is at the Hale Koa hotel on Waikiki. It's a military hotel, but anyone can dine there.
Every Friday night, the Hilton Hawaiian Village hotel on Waikiki shoots off a fireworks show. It was so magical- leaning against a palm tree with my family, listening to the ocean, watching the fireworks. So great!
About 15 minutes north of Aulani is Pokai Bay- dolphin tours are run out of there. We stayed at another military cabin type place there and we could see the dolphins breaching from our cabins.
Near Matsumoto's shave ice in Halewei is this cool soap factory- I think it's called North Shore Soap Factory. It's in an old sugar cane factory- neat products, and then there was a coffee roasting place/art house next to it. We have fond memories hanging out there one afternoon.
DH and I are going for four nights in January- we got Aulani for 2 nights and will stay at Hale Koa for 2 nights. I have been to all 4 main islands, and I really like Oahu the best. However, I haven't been to Maui since I was 12, so I don't trust my judgement with that island. Oahu is so incredibly diverse- nightlife and great shopping in Honolulu, and great snorkeling/quiet places just 20-30 minutes away.