The Pacific Northwest is generally a safe place to visit, and Portland has one of the lower crime rates of major U.S. cities its size. You'll often see people in coffeehouses leave laptops and bags unattended when they head up to the counter for a refill. That said, it's better to be safe than sorry, so keep an eye on your belongings in public places.
Always lock cars. Try not to leave any valuables inside the car; if you must do so, put them in the trunk. Car break-ins do happen in Portland, even in seemingly peaceful residential areas, and bike thefts are an unfortunately prevalent problem here, too.
The Great Outdoors pose the most dangerous element of the Northwest. Avoid hiking alone, especially in very remote areas, and make sure you bring enough water plus basic first-aid items. If you're not an experienced hiker, stick to tourist-friendly spots like the more accessible parts of national and state parks.