Go to Cannon Beach, be there early.
This weekend is a negative tide. Friday -1.8 feet at 7:56 am, Saturday -1.7 at 8:44 am, and Sunday -1.4 at 9:30 am.
Cannon beach has some of the best tide pools on the coast, plus the incredible Haystack Rock (which the tide pools are located around). This is a perfect weekend to go check them out. Be there a little before low tide to get parked and walk down to the beach. Trust me, this is absolutely one of the essentials. Tide pools can show up in many conditions, but a negative tide, especially one this low, will show you the best possible experience.
Just north of Cannon Beach is Ecola State park, which famously was used in filming for The Goonies and Free Willie. Further north at Astoria there is a bunch of historical sites from the Lewis and Clark expedition, the naval gun batteries from WWII that protected the Columbia, and Sunset Beach. You can also see the remains of the 1950 wreck of the Peter Iredale.
Going south from Cannon Beach you reach Tilamook, which is a fishing and dairy town. Also the largest town on the coast. Nearby is Tilamook Bay, where I go clamming. There is also the start of the Three Capes drive, which starts at Cape Mears on the west edge of the bay, which has a lighthouse and the famous ‘Octopus Tree’. You can continue south to hit Cape Lookout and Cape Kiwanda.
Cape Lookout is a thin cliff that juts out almost a mile into the ocean, several hundred feet high. Great views. Cape Kiwanda is a large sand dune you can walk all around, with some seal haul outs as well as bird nesting sights. Great for wildlife viewing and just exploring.
There is also further south Otter Rock, which has The Devils Punchbowl. A really cool sight to see at low tide, as you can walk into this giant basin that fills at high tide. You can also go to Newport and visit the aquarium there, as well as Seal Rock. By that point you are about 2.5 hours from Portland, so plan accordingly.
Another great tide pool spot is Yaquina Head down by Newport. There is an old quarry which was turned into an artificial reef, and has some of the best tide pools out there, and is part of the Yaquina Head Marine Garden.
Anyhow that is only a sampling of the possibilities. I’m going to cross post this to the Cascadia thread, but if you want any tips, ideas, or even want to learn clamming so you can dig clams by Garibaldi, well I am your huckleberry.