In addition to the usual suspects (Gold Creek Pond, the Iron Goat Trail, and Rainy Lake), there were lots of good, fresh ideas, including the following (no order, no or minimal editing, no endorsement, probably not comprehensive -- I think more comments have already appeared!):
Hurricane hill in Olympic National Park‚ (they just paved the whole trail a few weeks ago)
Big Meadow Loop at Hurricane Ridge
Trails at Hurricane Ridge in Olympic national Park (paved, but some steep)
Olympic Discovery Trail
Quinault Forest Nature trail
Picture Lake near Mt. Baker! (ADA, paved)
There's about 150 yards of paved trail at artist point by Mt. Baker ski area. Not much but excellent views with lots of other little places nearby in the same area.
Shadow of the Sentinels by Mt. Baker (boardwalk) through old growth forests with several view points and picnic spots. All ADA accessible.
If you do head up to Mt Baker make sure you swing at through boulevard Park is completely paved and or wheelchair accessible and a gorgeous walk on the waterfront!
Paradise, at Mt. Rainier, has some paved, but steep trails, such as the Skyline Trail to Myrtle Falls
Carbon river on Mt Rainier (first 5ish miles, make sure the chair has big wheels for this one)
Chambers Bay Beach Access, University Place, WA 98467
411 S 348th St, Federal Way, WA 98003
(Chambers bay has a pair of Osprey that have a nest)
Deception Falls. Off of Highway 2 (may have closed gate, so no access)
The Foothills Trail that runs from Puyallup through Orting and on to Buckley (paved)
Cedar river trail
Soos creek trail
Magnuson park waterfront trail are all wheelchair friendly.
Greenwater lakes trail
Sammamish River Trail i(paved)
Centennial Trail from Snohomish north to Arlington is paved.
Rhododendron Trailhead just north of SR 92 at Lake Stevens and heading north.
Nisqually wildlife refuge (wooden boardwalk)
Old Sauk River trail (ADA, gravel loop) on Mt Loop Highway
The Ho River trail, on the Olympic Peninsula (paved for the first couple of miles, flat)
Anacortes, Fidalgo island : 3 different paved trails with water views, Tommy Thompson trail that goes over Fidalgo bay ,the Guemes Channel trail by the ferry terminal, and the loop at Wa park
Padilla bay trail in Bow (paved)
Part of Rockport State Park.
Tradition Lake Loop (Around the Lake Trail) of Exit 20 (High Point) on I90 (wide gravel path)
Puget Power Trail (not paved, but a hard-packed access road) at High Point, exit 20
Friends Landing, Montesano, Wa.
The Willapa National Wildlife Refuge (ADA, 0.3 mile round trip)
Franklin falls had a stroller/wagon trail that ends with a little sort of picnic area by the river.
Ebey waterfront trail
The Theler Wetlands trails in Belfair
Mima Mounds in Capitol Forest (1/2 mile ADA accessible path)
The Big Four Ice Caves has a paved/boardwalk pathway and picnic area on the Mountain Loop Hwy. It is not accessible (steps) after the boardwalk.
Des Moines creek (paved all the way up along side the creek. And not too steep going up). There is the marina as well with the boardwalk out over the water.
Miles of trails along the Skagit County sound are paved and quite flat.
Erinswood, the new ADA trail in Index at the bottom of Heybrook Ridge!
Snoqualmie Valley Trail ( hard-packed gravel)
John Wayne Trail / Iron Horse Trail (grave), bring headlamps for tunnels
Coal miners trail in cle elum (road, hard packed gravel) connects Roslyn, Cle elum, and Ronald
Chehalis Western Trail through Olympia and down past Tenino (paved and flat)
Stimpson Forest Preserve , Bellingham, is full of great access trails.
Bradley Lake in Puyallup is paved all around.
Nathan Chapman park on 144th in Puyallup ,
Cross Kirkland Corridor between Bellevue and Kirkland. (mostly flat and wide)
I-90 Trail. It follows along I-90 from Seattle to Bellevue (paved)
Whistle lake in Anacortes (Not paved but very flat and wide)
Point defiance has a beautiful park with a ton of accessibility
Bpa trail in federal way (paved)
Downtown Issaquah Rainier Trail.
Deschutes Falls https://www.co.thurston.wa.us/parks/parks-deschutes-falls.htm
North Creek County Park, wetlands in Bothell
Bridal Trails State Park in Kirkland and Bellevue
Mt. Grant on San Juan Island
The Mount Baker Snoqualmie National Forest has videos of many of their accessible trails, narrated by a man using a wheelchair so you get a better idea of just how accessible they are. https://www.fs.usda.gov/detail/mbs/recreatio‚Ä¶ See More
Mt. Baker-Snoqualmie National Forest - Recreation