I would definitely rent a car. Unless you are beach bums or tennis pros, you won't want to stay at the resort every day. Plus, you'll likely want to go out for dinner several nights of your stay.

IIRC, KOA car rentals are fairly inexpensive. I believe we got a full size (Chevy Malibu or similar) for about $30 per day + tax (search the Avis forum to get some amazing discount codes). If you want a fancier car (ie convertible or Cadillac), I would figure on about $80 per day.

Valet parking was free when I was there, though I believe that may have been part of a promotional package. In any case, self parking is very convenient and totally free.