If you don't want to part with your belongings; don't leave them in your car. It is quite common to have your car broken into here so be careful. Leave items in the trunk and not visible to passers by.
Although we didn't encounter any problems (this was mainly because we were extremely careful and heeded the warnings) we saw plenty of evidence of other less fortunate tourists.
Everywhere, in road side parking spots we encountered broken car window glass where some unlucky soul had left his/her wallet/camera/equipment while they went for a hike.
If you must leave your vehicle...take all your personal belongings with you.
If you're traveling with luggage, best to stay within eye sight distance to your vehicle.
You can always return to the spot after having checked into your hotel room. Leave all valuables back at your hotel!
Do not leave cameras/wallets/purses in your car. We even made it a point to leave car unlocked.
One must always have an eye for car break-ins in Hawaii. Some tourists even leave their cars unlocked, and their glove compartments open to show that there is nothing to steal.
That does not always deter the thieves however and you might find your locks smashed anyway just out of frustration I guess.
Often tourists return to their cars to find that their trunk lock has been punched in. So take nothing to any of the beaches or remote locations that you could not do without.