I do think the Arizona memorial is worth it. The best time to go is early in the morning (right when it opens) to avoid lines. Tickets are passed out as you arrive indicating the time the boat will take you to the memorial. Before the boat departs you will be admitted to watch a short film (20-30min) about Pearl Harbor. You may want to check the web or call ahead to confirm the hours, but my recollection is that the memorial is open 7 days a week. If you are into flea markets you can after the Arizona you can walk across the street over to the stadium and go to the flea market there - a good place to pick up cheap souviners,t-shirts, etc.

There are so many wonderful beaches and areas on Oahu that aren't in Waikiki. I'd suggest spending some time on Saturday afternoon exploring either the north shore/Haleiwa or going to the windward coast. The beach at Kailua is really nice. It is also a good spot to watch wind surfers, kite surfers, and rent a kyack if you have time.

You'd still have Saturday evening to take in the scene in Waikiki (mostly expensive shops and things geared towards tourists, high rise hotels, etc.)