Whitman Mission National Historic Site - The 1847 Whitman "Massacre" horrified Americans and impacted the lives of the peoples of the Columbia Plateau for decades afterwards. See under Walla Walla#Get out.
For our purposes, the Columbia River Plateau region consists of Adams, Benton, Douglas, Franklin, Grant, Klickitat, Lincoln, Walla Walla, and Yakima Counties.
Umatilla National Wildlife Refuge, a 14,000-acre wildlife refuge straddling the Columbia River in Washington and Oregon. Entry points are in Irrigon, Oregon, and off Highway 14 in Washington
US Dept of Energy Hanford Site Test Facility. One of the major nuclear weapons production sites in the United States Tours happen six times a year and depart from Richland.
Crown Point Overlook State Park, (From state route 155 in Grand Coulee, take the turnoff to Bridgeport (state Route 174), drive two miles north, look for a sign 'Crown Point Vista'). The best spot for viewing the Grand Coulee Dam.