Must do's for us (a.k.a things we would do again) would be a beach day in Kailua, a day exploring/hanging out in the North Shore, a tour at Kualoa Ranch, and people watching, shopping, dinner/drinks in Waikiki.
If this is your first trip, I would add Pearl Harbor as a must do. We did Diamond Head as well which is a popular tourist thing, but not something I'd consider a must do. We also did not care for the Aloha Swap Meet - same cheap touristy trinkets you can find most anywhere, lots of booths had the same things, etc.
For eats, we enjoyed Monkeypod across the street from Aulani. Lots of great options in Waikiki - House Without a Key is often recommended. Loved Kono's in the North Shore, and they also have a location in Kailua. The buffet lunch at Kualoa Ranch was fantastic, though I'm not sure if they're currently offering it with the pandemic. We also visited Anna Miller's which is a cheap diner stop for a good breakfast or pie! Also loved Leonard's malasada truck. Its an easy stop on the way back to Aulani, and easier/less busy than going to the Leonard's location.