When we were in Kauai a few years ago, we had a standard Buick for a rental car. The parking lot at Kee beach was very rough and somewhere I lost two hubcaps (don't really know if they were "lost" when I rented the car), but I found a matching one on our trip to the "Grand Canyon of Kauai" and threw it into the trunk. The car also had a lot of "dings and dangs" on it - I don't think I was responsible - but I never seem to check my rental cars the way I should. Nothing was ever said about it.

FWIW, the cars will get filthy dirty in Kauai with all of the "red dirt".
Enjoy!
DD