I say go to Hawaii. Many feel Oahu is their least favorite island because Waikiki and Oahu in general are more crowded, but there is PLENTY to do and see. First of all, the lagoons by Aulani in the Ko Olina Resort area are great! They have little to no surf, so the kids can play in the water with ease! There are 4 lagoons in that area that you can utilize, and minimal parking for non-resort guests, so they are never really that crowded. If you rent a car (which you had in your calculations) you can see so many of the sites! There is Hanauma Bay for snorkeling, Makupuu Lighthouse for an easy hike with a beautiful view of the little islands to the east and possibly Maui in the distance, Diamond Head crater hike with views of Waikiki, the North Shore beaches (in February there are still giant waves, so there will be surfers and possibly a surf competition). Haliewa Town on the north shore has some great "hole in the wall" type restuarants and the famous Hawaii Shave Ice stand Matsumoto's. Ko Olina has a great luau and then along the leeward coast there is the Polynesian Culture Center that puts on a great night show. Then there is the history- Pearl Harbor and the USS Missouri are great tours. For cheap eats, grab a "plate lunch" for around 5-6 bucks and you'll be full all day! Sure you can spend a lot on food at the grocery stores, but the normal restaurants in my experience living there were not that much higher than the mainland (minus the resort area itself). In my opinion, if you have just one week, there won't be enough time to see all that Oahu has to offer AND travel to another island. Best of luck in your decision! Happy New Year and Happy Planning!