There’s a cool pill box hike in Lanakai. It’s an amazing view that overlooks the beach when you get to the top of the pillboxes. It’s very short, will probably only take about 30 mins to an hour. My family likes to get up and do the pillboxes about 8 in the morning before it gets crowded. You’ll get hot and sweaty then hike down to the beach and jump in the water.
I think hiking diamond head is overrated but you may consider doing it if you aren’t going back for awhile.
some people think snorkeling at Hanauma bay is overrated but I always enjoy it. I could spend hours floating around out there.
I don’t know where you’re staying in Oahu but I recommend not spending much time on your hotel beach. Drive around and find a bunch of prettier beaches like Lanakai, Kailua, multiple beaches on the north shore, Holona (sp?) cove, etc. Waikiki is crowded and eroding away. Ko Olina is good for little kids but otherwise pretty boring (no snorkeling, no surf, no views).
A few of our usual restaurants were shut down when we were there in March due to COVID fear. Not sure what’s open now.