Yup, you must hit up Ala Moana! Seriously one of the best malls I've ever been to (I'm from Hawaii, but have been to many other malls around the country -- not impressed).
I'm not sure if you're looking to do anything adventurous, but you could go parasailing or hiking. There are many hiking trails on Oahu (I'm sure on Maui too, but I've never been there). I'm not a big adventurer, so I liked hiking Manoa Falls (gorgeous scenery) & Diamond Head (great view at the top, essentially next to Waikiki if that's where you're staying).
Wow soo lucky! I just came back from Hawaii (Oahu) about a month ago and I loved it! It made me consider moving there after college.
Anyways, go to the ala moana mall like everyone suggested. I also liked snorkeling at Hanauma Bay. Go hiking in the manoa valley to the water fall. If you get a chance, hike by diamond head. Theres so much shopping to do in Hawaii especially in Waikiki. I also loved Kailua beach (im not sure if thats the right name and/or if im spelling it right). I loved the marketplace in Waikiki so you can check that out. If you can, go to the Polynesian Culture Center. Theres a luau there and other day activities/shows. I also went on a catamaran dinner cruise which was fun. The view of waikiki was beautiful.
As for maui, i went there 2 years ago but I don't really remember what there is to do. I was there long since I was on a cruise when I went there. I just remember going on a tour that was set up through my cruise line.
When I was in Oahu 2 years ago, I went to pearl harbor, the dole plantation and we went to the paradise cove luau which I loved.
I would second the suggestion to visit the Arizona Memorial in Pearl Harbor--an indelible part of world history. I knew a survivor of the attack.
About the road to Hana, if you are prone to car-sickness (it's hairpin turns pretty much the whole way), sit in front, take some salty snacks and/ or dramamine (can't drive if you take the medicine). Even my husband who never got motion sick in his life, felt bad after our trip. We took turns sitting in the back seat.
^^ I got so car sick on that drive to Hana that I didn't really enjoy much of the sight seeing along the way. However, drinking carbonated beverages and eating ginger snap cookies seemed to help settle down the motion sickness/nausea a bit for me. To this day, I'm still hesitant to ever attempt to do that drive again.