Oahu-only Advice:
-Pearl Harbor/ USS Arizona Memorial
-Snorkeling in Hanauma Bay
-Kailua or Lanikai Beach Park. Kayak/canoe out to the Moku Islands
-Eat at a shrimp truck up near Kohuku/Turtle Bay. Amazing food.
-North shore. Pipeline is a must watch if it's breaking. If you're extra lucky and Waimea Bay is breaking, check that out for sure as well.
-In Haleiwa, they have Hawaii Shark Encounters. They'll take you out about 3 miles where they have a setup with shark cages. It's absolutely awesome. You might get a little seasick if the waves are choppy, but that's only on the boat. In the cage, it's awesome. Buy an underwater camera if you do this.
-If Makaha is breaking on the west side, that's a cool place to watch the surf action also. Best Hawaiian Ice ever is there, too.
-There's also a Circle Island Tour company that does Hummer vehicle tours of TV/movie set locales. So if you're a fan of Lost or Hawaii 5-0, that's a fun tour.