Planning a trip for my family of 5 (kids are 21, 18, 15) to Kauai sometime next year between end of June and early August. Never been there before. Any recommendations on nice but affordable places to stay (airbnb, beach house, etc.)?

Planning to have a rental car to hit the north end and south end and will likely buy groceries for most meals (breakfast/lunches).

Also, what attractions are "must see" or activities are "must do" for a family with my kids ages?