In general, Hawaii is a very safe place to visit. For travelers, the two main concerns are car break-ins (common) and natural disasters (rare but serious).
Pakalolo (marijuana) remains a billion-dollar underground industry, and the use of 'ice' (crystal methamphetamine) is a constant issue. The 'ice epidemic' has abated somewhat recently (as a result of enforcement efforts), but ice-related crimes continue and social-service agencies still struggle to provide treatment for addicts.
The main scams in Hawaii involve fake activity-operator booths and timeshare booths. Salespeople at the latter will offer you all sorts of deals, from free luaus to sunset cruises, if you'll just come to hear their 'no obligation' pitch. Caveat emptor.
The islands are notorious for rip-offs from parked rental cars. Do not to leave anything valuable in your car, ever. If you must, pack things well out of sight before you arrive at your destination; thieves wait and watch to see what you put in the trunk. Many locals leave their car doors unlocked to avoid paying for broken windows. Otherwise, the most common problem is being hassled by drunks on beaches and in campgrounds, mainly at night. Overall, violent crime is lower in Hawaii than in most of mainland USA.
Hurricanes, tsunami and earthquakes all occur in Hawaii, sometimes to devastating effect. In 1992 Hurricane 'Iniki wrecked large swathes of Kaua'i, and in 1946 the largest tsunami in Hawaii's history killed 159 people and caused enormous damage. On average, tsunami have occurred once a decade over the last century and killed more people statewide than all other natural disasters combined. In 1948 Hawaii installed a tsunami warning system, aired through yellow speakers mounted on telephone poles around the islands. If you hear tsunami warning sirens, head for higher ground immediately. The front sections of telephone books have maps of tsunami safety evacuation zones. Usually, there is ample warning prior to hurricanes.