Now is a good time to remind people that if you're not a champion surfer with a bleached-blond 'do and a board under your arm, you have little business dipping even a toe into the waters of most beaches on Oahu's legendary north shore. This is because the reason those surfer dudes are there right now is the enormous waves and gnarly conditions that the islands get in the winter.
Sunset Beach, Pipeline and the other great North Shore surfing beaches are strictly for pros right now, and unless you can swim like Duke Kahanamoku, you should use them just for sun-bathing and people-watching. The exception to this is Waimea Bay Beach Park, which has a nice, protected lagoon where kids can safely splash around, even as the big waves are pounding the shoreline around the corner.
Waikiki Beach is generally benign all year round, but it is always crowded, and it would be a pity to go to Oahu and not experience the diversity of great beaches on the island. I always stop at Kailua Beach Park on the windward side of the island (over the Pali Highway in your rental car), not only because I once lived a block from there but because it's a great mix of kite-boarders, locals, families and kids, with a gentle shore break in the reef-protected water. Just down the road from there is the community of Lanikai, with one of the great white-sand and powder-blue-water beaches anywhere. And if you keep going east to the village of Waimanalo, you'll find Bellows Beach to be a long, protected beach that is utterly secluded and is a great place to learn how to boogie-board without getting smashed to bits.
Sandy Beach and neighboring Makapuu Beach, on the other hand, can be rough and crowded. I speak from experience, having been utterly thrashed when I foolishly tried to apply my innocent boogie-boarding skills there. Whichever beach you choose, keep in mind that the rest of us are all totally jealous that you're in Hawaii and we're not.