Kalapaki Beach is a lovely crescent-shaped sand beach in front of the Kauai Marriott Resort and the Kauai Lagoons championship golfcourse.
The Hoary Head mountains and Haupu Ridge that protects the bay can be seen from the sand and the beach faces the shimmering Nawilwili Bay and Harbor itself.
You can find volleyball, sailboat rentals, catamaran cruises, and surf lessons nearby and there are some nice swells so it's a great spot for swimming, surfing and boogie boarding (with occasional strong currents and dangerous shorebreaks to look out for).
A popular picnic area is in the adjacent Nawilwili Park and facilities can be found in the nearby Anchor Cove Shopping Center.
Although there's no Kalapaki Beach sign, follow the "Shoreline Access" sign just past the Marriott to find one of the best east coast beaches on Kauai. (Or turn right off of Nawiliili Rd. onto Wa'apa and the harbor will be on your left).