You'll enjoy the waterfalls and riverfront in Wailua Homesteads. You might want to make time for a stop at Kadavul Hindu Temple or Kaua'i's Hindu Monastery.
If you're spending some time in Wailua House Lots, Lae Nani Beach and Wailua Beach are top sights worth seeing.
While there might not be top attractions in Kapahi, you can explore the larger area and discover places like Ho'opi'i Falls and Kealia Forest Reserve.