IMO, Kailua has the most beautiful beach we saw during our 16 day/3 island trip to Hawaii! It's on the other side of the island from Aulani, so about a 45 minute drive. It's not too far from Kualoa Ranch if you plan to visit there, which I also recommend! But Kailua is a quaint little town with some cute shops and restaurants, and there was a beach park with some parking. We just stopped by but had I known, I would have planned a day there. If you listen to the Dis Unplugged podcast, John & Kevin also recommended visiting Kailua during their Aulani series last year.
We also loved the North Shore and the town of Haleiwa. We stumbled upon Laniakea Beach in that area which was small and busy because of all the sea turtles there. But along that whole stretch are other beautiful, less crowded beaches as well. And Haleiwa was a great place for shopping, dinner, and a shave ice stop!
I have photos of both Kailua and Haleiwa/Laniakea Beach in my trip report which is linked in my signature!