Our family is big on food, so we love visiting any farmer's market (though my friend who's a local recommends Kaka'ako as it's closer to Aulani and has gotten a lot more vendors over the pandemic), poke at Foodland, and any garlic shrimp truck in North Shore, though I know Giovanni's is the most popular (the spicy shrimp is no joke though, our family eats spicy food and we were dying). We stayed closer to the SE side of the island when we were there, but also really enjoyed Moena Cafe, Cinnamon's, and Uncle Clay's House of Pure Aloha.
For non-food, an easy walk to nice sights is Makapu'u Lighthouse. I really enjoyed Hanauma Bay for snorkeling and general beach hangout, but I believe as a non-local, you need to make reservations in advance now. We went right at 7am as we were still on Pacific time and it got pretty crowded by 8am on a Monday during a long weekend, so I'd recommend going early if you do that. If you don't want to do Waikiki, Kailua Beach on the east side is popular with locals and had similarly soft white sand. Then on the North Shore, especially in winter, just watching the surfers ride a wave in from really far out in the ocean was very fun to watch.