References such as “Hike 7” refer to hikes in my book Day Hiking: Oregon Coast.
Cumulative mileage figures are for the section described here.
Many times I have day hiked to the top of Tillamook Head and admired, but not stayed in, the sweet little shelters there for backpackers. Nor did I spend the night there on my through-hike of the OCT (timing didn’t work out; I was too eager to make miles). It almost seems worthwhile, if you’re starting in Seaside, to plan a scant 8-mile day just to spend a night in those shelters.
Picking up the trail in Seaside at 12th Avenue and N. Holladay, cross the river (restrooms and water in city park here) and go west a few blocks to hit the beach at the north end of Seaside’s Promenade. Walk this busy beach (not so busy in this photo by M.O. Stevens; it's busier further south, near the lifeguard towers) south toward Tillamook Head. Approaching the Lanai Motel, at the spot known among surfers as The Cove (where the beach starts to run out), climb up through the cobbles to Sunset Boulevard and follow it south (passing restrooms at little Seltzer Park, on the east side of the road across from a roadside parking area) for a mile or so until the road ends (3.5 miles) at the trailhead for Tillamook Head (Hike 7). The trail switchbacks, steeply at times, for the first 1.5 miles but then becomes a gentler ascent, eventually rolling gently up and down to a primitive campsite (7.9 miles) with vault toilets, a covered shelter with picnic table, and three three-sided sleeping shelters. (You will want to hang your food here, not from bears but from rodents; since this is a well-established campsite, I understand the critters here have a well-established habit of raiding backpacks.)
(Tillamook Rock Lighthouse from Tillamook Head, by Randall Henderson.)
When you’re ready to continue—whether it’s that same day or the next morning—continue south on the Tillamook Head trail just 0.2 mile to a junction. The lefthand route (“Clatsop Loop”) follows an old road and thus has a gentler grade (and is 0.1 mile shorter) than the righthand footpath. Both trails end at the parking lot above Indian Beach (9.4 miles). Cross the parking lot and pick up Hike 8 to Ecola Point (10.9), then cross the top of that parking lot to pick up Hike 9 near the restrooms. After 1 mile the trail splits. If the tide is quite low and you don't mind getting a little wet, you could continue down to Crescent Beach (Hike 9) and squeeze between the end of the headland and the nearest offshore rock separating Crescent Beach and Chapman Beach north of Ecola Creek. (Even if the tide is out, deep "crab holes" form here and might require some pretty deep wading.) Otherwise, rather than going right to Crescent Beach, go left 0.1 mile to where the trail meets Ecola Park Road (12 miles). Walk down the road just 0.25 mile to Ash Street (at 8th Avenue), follow Ash south (becomes a steep pedestrian path) to 7th Avenue, and take 7th Avenue 0.5 mile west to the beach. Continue down the beach 0.75 mile to the mouth of Ecola Creek (13.5 miles), which you can wade in late summer/fall. (If you prefer to keep your feet dry, stay on Ecola Park Road to Elm Street, follow it over Ecola Creek, then follow the main road to the beach access at the end of Third Street.)
Don’t think about camping on the beach here—the Cannon Beach Police patrol the beach nightly and will roust you out—but there is plenty of lodging in town, and you could pitch your tent legally, for a fee, at someplace like Sea Ranch (searanchrv.com), just east of the bridge over Ecola Creek.