Just 5 minutes out from a side road from the airport in Kahalui gets you to a happening beach that's pracitically unknown to tourists. But some of the world's best kite-boarders are all over it, and they provide a colorful spectacle. Strapped to a surfboard and harnessed to a kite (that has enough lift to get them airborne without going bye-bye) these guys and girls frolick and fly over breaking waves, with the West Maui Mountains in the background looking like another island.
Big Kanaha Beach Park has and arboretum of huge beach trees and picnic pavilions that draw locals and families. Windsurfers also love Kanaha, also the real action for that sport is just up the coast at Hookah, er, Hookipa Beach Park.
This is windward (east) Maui, so whitecaps are common, but big waves are held in check by long Spartan Reef, which is well off-shore.
Head to the right (as you face the water) and you will find some of the best little sandy nooks to spend a quiet day at the beach—not easy to find on Maui. You can walk around a palmy point, passing beach homes just inland, and reach the long stretch of open beach at Spreckelsville. A near shore small reef creates a sweet keiki (kids') beach.
Maui Trailblazer has more details on this non-touristy run of coast from Paia to Kahalui.