Consider booking a sunset cruise around Waikiki on your first day. This is a super low-key activity, perfect for getting out there to do something without doing too much.
While visiting Oahu is about soaking up the sunshine and enjoying the beautiful beaches and Hawaiian culture, its important to visit places like Pearl Harbor to pay your respects to the people who lost their lives here.
Your whole 3rd day on this Oahu itinerary is mostly going to be a beach day spent on the North Shore, so make sure you pack everything you'll need for the day in your rental car before the early start.
Day 4 of your 5-day Oahu itinerary is going to be spent on the eastern shore of Oahu and it starts with a road trip through the Pali Highway towards the eastern coast of the island.