Waikiki is fine for what it is. But for a date where you can actually hear each other and enjoy the place you're in, here's where Oahu residents actually go.
Kailua Beach Park on the Windward Side is genuinely one of the best beaches in the United States — calm water, white sand, turquoise color that doesn't look real in photos until you're standing in it. It's a 30-minute drive from town over the Pali Highway, which means it's never as crowded as Waikiki, and the town of Kailua itself has good coffee and food options for before or after. Best on weekday mornings.
On the North Shore, about an hour from Honolulu, Laniakea Beach is where Hawaiian green sea turtles regularly come ashore to rest. It's genuinely wild to stand next to a 400-pound sea turtle on a beach, and sharing that with someone tends to be memorable in the way that restaurant dinners usually aren't. Don't touch the turtles (it's illegal and wrong) but do stay and watch.
A short walk or bike ride from Kailua Beach, Lanikai is a residential neighborhood beach that requires a little more intention to find — which keeps it less crowded than the main beach parks. The Mokulua Islands offshore are kayak distance, and renting kayaks from two good kayaks in Kailua makes for an excellent half-day date if you're both comfortable on the water.
On the eastern tip of Oahu, Makapu'u Beach has one of the most dramatic settings on the island — big waves (don't swim unless you know what you're doing), a lighthouse trail with panoramic views, and a feeling of being at the edge of the Pacific that's quite different from the sheltered bays on the other sides of the island.
For a date that doesn't require a long drive, Ala Moana Beach Park is the local's alternative to Waikiki — a few hundred feet away but a completely different crowd and vibe. Calm water, sunset views toward Diamond Head, and the fact that you're sharing it with actual Honolulu residents rather than tourists makes it feel genuinely different.