Unfortunately, Hawaii has a big Crystal Meth Problem and due to that, the number of rental car break-ins on all of the islands in Hawaii is actually pretty high. Other helpers we´ve met during our workaway on the Big Island got all their luggage stolen on O´ahu. When we went to Maui for a week, we didn´t want that us happens the same. We always parked at busy and public places and hided all our stuff in three steps:
Also, we carried the most important things like passport, wallet and cameras with us in a daypack. If you need more useful tips, check out the detailed advices on this blog.
Have a safe trip!