While the Diamond Head hike itself may be too difficult for young children, older kids will be thrilled at the chance to walk through a volcanic crater. Just remember to take plenty of water and sunscreen as there is very little shade on the trail. The hike along the cliffs is much less demanding, while the lookout points - including one dedicated to Amelia Earhart - still offer fantastic views of the Pacific Ocean.
The beach at Kuilei Cliffs is ideal for surfing, and you can hire equipment at various places around the island. Look out for the seals which can often be seen basking here but make sure you don't disturb their sleep!
Diamond Head Beach Park is a popular choice if you want to get away from the busier beaches in the more popular parts of O'ahu. Cromwell's Cove is a lovely sandy beach, where you can enjoy a peaceful dip in the Pacific.