Take a drive up HWY 21 and ride the Keller Ferry across the Columbia River. A ferry at this location has been in operation since the 1890s and has been controlled by WA state since the 1930s. Keller Ferry was the first ferry owned and operated by the State of Washington and is still the only one in Eastern Washington. The ferry is free and runs from 6am to Midnight, 7 days a week. Cars, RV, Trucks, and Pedestrians are all allowed.
The cache should be an easy one at the rest stop on the north side of the river. You're looking for a small container stuffed full of goodies including an unactivated mermaid trackable for FTF.