Astoria would def. be a good starting point. Besides Fort Stevens (very interesting), there's:
The 125 ft tall Astoria Column which sits on top of 600 ft Coxcomb Hill. Terrific 360 degree panoramic view. Take your own balsa wood glider or buy one at the gift shop and launch it from the tower.
Numerous movies were filmed in and around Astoria and on the coast like Free Willy and The Goonies. The Oregon Film Museum is in Astoria in the old Clatsop County Jail and has exhibits.
If you could make the timing work out, Seaside has the annual "World’s Largest Amateur Beach Volleyball Tournament" in August. We happened to be there last year when it was on and was fun to see some of it.
If you were to hop over the Astoria-Megler bridge to WA, there's Long Beach which has the longest beach in the US (28 miles) - lovely little town to visit in the summertime. IIwaco is just down the road and there is a Saturday market on the boardwalk and a few places to stop and eat there. Cape Disappointment State Park (1800+ acres) is interesting to drive around and they have campsites, yurts and cabins.
We stayed in Newport last month for a week and enjoyed it. Was rather cool and cloudy and not much fun for sitting outside. They have Nye Beach, a small historic area near the ocean and the Historic Bayfront along the river which both make a great visit.
Neskowin just north of Lincoln City is a quaint interesting place to visit. Very small and has a bunch of summer cottages next to the ocean, many of them old. You can walk through it but have to park nearby. Recommended!
We went on a horse ride along the beach in Pacific City a few years ago. Great fun but hard on an aging body...
You'll see lots of tsunami warning signs along the coast. Make sure your running legs are in shape and have your sneakers handy! :)