Wynoochee Lake — Lower central part of the Olympic Peninsula. Supposedly lots of paddle/hike-in only camping spots on the west side.
Kitsap Peninsula National WaterTrails (KPNWT) offers 371 miles of shoreline for a variety of water-related activities – kayaking, SUP, boating, scuba diving, fishing, wildlife viewing and leisure strolls along public piers and beaches
Kimple Beach to Point Doughty [WWTA 1, info sheet] for an overnight camp. Or start at West Beach Resort, immediately north of Kimple Beach ($10 per way launch fee + $7/day parking). Reportedly an easy 2.5 miles, if conditions are good. No access by land, and too shallow for most power boats. 3 non-reservable camp sites up a steep dirt trail with a rope assist; pit toilet is reportedly in poor shape.
Columbia River from Bonneville Dam (Fort Cascades Boat Ramp) to Hayden Island is ~37 river miles per a facebook post, and it’s around 50 miles if you keep going and turn up the Willamette to downtown Portland?
Lower Columbia Water Trail stretches 146 miles from Bonneville Dam to the Pacific Ocean. The linked page includes a map with launch spots, camping spots, places to get food, and points of interest.
Cowlitz River from Blue Creek Boat Launch (requires Discover Pass or WDFW pass) to Toledo Boat Launch ($5 launch fee; restrooms; close to downtown) is around 8.5 river miles; supposed to be class I (maybe a class II stretch at first turn below the launch?) easy float or paddle.
Lewis River from just below Lake Merwin to the Columbia River
Launch at Merwin Ramp just below Merwin Dam. There are reportedly some fast-moving sections (class 1 rapids?) in the 5-6 miles downriver from this spot.
An alternate is to launch at Paradise Point State Park on the east fork of Lewis River, much closer to the Columbia. Reportedly easy paddling up and down river from here. (Also a possible take-out point when paddling from up the main fork.)
Pend Oreille River Water Trail — 70 miles of the Pend Oreille River, starting in Oldtown, ID, then follows the river north, immediately crossing into Northeastern Washington, all the way up to Boundary Dam, one mile short of Canada. Some portages required; some optional due to rapids. This site has both “overview” maps (“North / Middle / South Reach single page”) and much more detailed maps split into pages (“North / Middle / South Reach individual pages”); the latter is much more helpful for finding rapids and required portages.
I saw a suggestion that you don’t want to attempt to paddle the lower stretch between Metaline Falls and Boundary Dam until late summer due to unsafe strong currents earlier. Various unnatural currents due to it being a man-made flooded valley, and paddling over the submerged Metaline Falls is quite dangerous. (Launch by the dam instead and paddle up from there. There are some cool falls on the west side, and a cave somewhere.)
The Palouse River below Palouse Falls, from Lyons Ferry SP at the confluence with the Snake River, up to Palouse Falls, is an interesting paddle. There are some rapids when you start getting close, too much to paddle against, requiring a portage. But at least earlier in the year that area is very overgrown, and with it being rattlesnake area I was very reluctant to go tromping through all this.
Spokane River Water Trail runs from a few miles across the border with Idaho, down to the confluence with the Columbia River. Not all sections are navigable, and some require whitewater skills, but much is broadly accessible.
Willamette River from Takena Landing Park (according to fb post, but a couple of alternate launch spots are nearby) to Wallace Marine Park. ~35 miles & 7 hours; suitable for sea kayaks, based on a trip report on facebook.
The Rogue River has sections of varying skill levels, up to class IV (or even V) or much mellower near the sea. PublicLands.com blog post
Clear Lake near Sisters, OR is known for dramatic underwater views thanks to very clear and cold water preserving a forest flooded thousands of years ago due to a lava flow.
See also Columbia River paddles filed under Washington State.
The Pend Oreille River Water Trail starts in Idaho, but immediately crosses into Washington. See the entry in the Washington section.