SW Terwilliger Blvd Parkway
This route features wide sidewalks for walkers, runners, skaters, skateboarders and the like, as well as bike lanes on both side of the Boulevard. It is largely wooded, so it's generally a bit cooler than walking/running downtown.
Some fun features include the staircase to the old Veterans Hospital and the Portland Health Course. There are also benches, several water fountains, and a bathroom.
If you're coming from downtown, start at Duniway Park, just south of PSU. Walk west, past the lilac garden, and then up the hill at the aptly named Terwilliger.
It's not a terribly long scenic byway: roundtrip from Duniway Park to Capitol Highway and back is about 5 miles. Proceed on to Barbur Blvd, and roundtrip, you have 6 and 2/3 miles.