Ulupo Heiau State Historic Site

This massive stone heiau tucked behind the Kailua YMCA is one of Oahu's best-kept secrets — most tourists never find it, and honestly, that's part of its magic. According to Hawaiian oral tradition, the legendary menehune built this ancient temple platform, and standing on it overlooking Kawainui Marsh (Hawaii's largest wetland), you can feel the spiritual mana that's lingered here for centuries. The site feels surprisingly substantial once you're there — reviewers are always amazed it's not a bigger attraction. Take the path around to the right and down to see the heiau from below near the taro fields for the full perspective.