There's much to be learned from the trees and shrubs at Keahua Arboretum. A place of science research, education and beauty, it'll bring out the tree hugger in you.
You'll be spoiled for sightseeing choice in Kapaa. Keahua Arboretum is a good place to begin here, but there are loads of other worthwhile spots to consider. Lydgate Beach, Coconut Marketplace, and Wailua Falls are not far from the botanical collection and will top off your day nicely.
