Since I'm often asked ....
1. Go to Waikiki Beach and take a surfing lesson. You'll be glad you did.
2. Get a rental car and explore the island, including the North Shore. Go during the day when everybody's at work.
3. Don't ever eat at a fast food joint like McDonald's. Instead, spend all your food money on the great local cuisine.
4. Go snorkeling. Forget the fins. You just need a mask and a snorkel.
5. Spend the day in Kailua (my hometown) -- beach, shopping, eating.
6. Do the Diamond Head hike (easy but great views of Honolulu).
7. Visit the Pearl Harbor Memorial.
8. Check out the Dole Pineapple Plantation.
9. Spend a day shopping at the Ala Moana Shopping Center.
10. Go to the Pali Lookout for the best views of Windward Oahu.