I can answer this one pretty well, as one of my relatives uses a power chair and we’ve gone many places together over the years. Are you looking for camping sites? Or parks and natural areas? Here are a few ideas:
Sunnyside Park and RiverBend Park are two very pretty campgrounds on mostly flat ground with lots of paved walkways. Sunnyside is right on the shores of Green Peter Reservoir, and RiverBend is on the banks of the South Santiam River. Both are very easy to get around in when using a chair.
Champoeg State Park outside of Newberg is another good option — a variety of camping options (tent, RV, cabin, yurt) in a mostly flat and level area with paved walking trails. Some of them are formally designated ADA-accessible, but all of them would be relatively easy to get around in a chair. It’s in a pretty part of the Willamette Valley, nearby to lots of great wineries, and has some interesting historic sites within the park.
The Oregon Garden is another great place to visit for folks using chairs — it’s an 80-acre botanical garden (bigger than Buchart Gardens in Canada!) — and the entire place is ADA compliant. There’s also an adjacent Oregon Garden Resort which has ADA compliant rooms you can book for the night.