Tucked all the way back in Manoa Valley, this is a gem of an arboretum operated by the University of Hawaii. Hike to a waterfall or sit and enjoy beautiful views of the valley. You'll also see an ethnobotanical garden, a Hawaiian hale and garden, and one of the largest palm collections anywhere—all within a park-like setting. Its educational mission means there are regular talks and walks with university faculty. Docents give 60-minute tours weekdays at 10 am (call ahead to reserve a spot). There are also self-guided tours.