This is controversial advice, but I follow it when I'm in Hawaii:

Lots of locals leave their car doors unlocked and windows open if they don’t have anything to steal inside. It’s unlikely a thief will go through the effort to steal your car; they just want an easy score. If they see your windows open and doors unlocked, they’ll assume you don’t have anything valuable inside. Many thieves will assume the owner of the car is local and will leave it alone.