My best advice - get away from Waikiki (tourist trap) & Honolulu (basically a big city) as much as possible.
The hike up Diamond Head is cool. Pearl Harbor is a must-do for any American. As we've had our kids with us on previous trips, we've done the zoo and the Sea Life Park. We did Germaine's luau based on Gabriel Iglesias' bit about it - cool to do once. Hanauma Bay is an over-used snorkel spot - it's crowded but has good facilities. I prefer more secluded, protected areas.
Just do some research, get in the car and drive. Way more to the island than Waikiki & Honolulu.
But never leave anything visible in the car. We got broken into on Maui when we were away from the car for less than 10 minutes.