Pearl Harbor leads the list in terms of popularity and it is the busiest attraction on Oahu. Pearl harbor is always sold out and it is necessary to either purchase tickets in advance for certain times of the day on a specific date you plan to visit or you can be one of the 1300 individuals that can show up early in the morning and wait in line in hopes of receiving a ticket for entry. Sometimes you will be turned away if they become sold out for the day in the early part of the morning. It is a bit of a hassle. The other thing you can do is to sign up for one of the tours on this page and when you purchase it you will be guaranteed entry for the visit with your tour group. It’s a bit safer this way.
The Polynesian Cultural Center is also very busy and visiting there can be complicated by the fact that it is located in the farthest part of the island away from your hotel in Waikiki. You can purchase a shuttle with the tour ticket or take your car out and enjoy a stop along the way at the famous North Shore beaches.