closest swimmable beach is not far from the airport (swimmable only because it's protected by a breakwater), however it's definitely not the nicest beach, and most probably not the one you'd want to spend the most time at. Both Hanalei and Polihale are awesome beaches (the latter being somewhat deserted simply because of its' remoteness, but it's worth the bumby drive through the cane fields to get to it!). And yes...in my opinion you'd want to rent a car for the whole week, as there's plenty to do and see outside of the hotel. Keep in mind that nothing is within walking distance of the Kauai Beach Resort.