Swim in the Columbia River at historic and scenic Rooster Rock State Park ($3 per vehicle), Exit 25 off I-84 near Troutdale. West of the parking area is the monolith for which the park is named. According to some sources, Lewis and Clark labeled the cucumber-shaped promontory on November 2, 1805.
Playing fields and a gazebo front a sandy beach on the banks of the Columbia. The water in the roped-off swimming area is shallow but refreshing.
A mile or so east is one of the only nude beaches officially sanctioned by the state (the other is Collins Beach on Sauvie Island [1]).
Links:
[1] http://www.moon.com/destinations/oregon/portland/sights/northwest-portland/sauvie-island