Kawainui Marsh is the largest freshwater wetland in Hawaii — about 830 acres of cattail, reed, and shallow water between Kailua town and the foot of the Koolau range. It was once a tidal estuary connected to Kailua Bay; later it became a managed fishpond under Hawaiian rulers, and is now a protected wildlife refuge.
A 2 km levee path along the eastern edge of the marsh is the best way to see it: open, flat, and quiet, with views over the wetland to the mountains and good chances of spotting Hawaiian stilts (aeo), coots (alae kea), and gallinules. Mornings and late afternoons are best for birds. The trail is also popular with local joggers and walkers.