For Oahu, everything SandyLS wrote. The north shore is an amazing place to be when the surf is really up. If you don't have time for the Polynesian cultural center, the Bishop Museum is great for Hawaiian natural history & culture. The walk/hike up DiamondHead is nice. A trip to the Punchbowl National Cemetery is a good addition to the Pearl Harbor military history stuff. Waikiki beach is great for people watching, but very touristy. Kailua is a nice option if you want quieter beach time.