Cannon Beach, nine miles south on US-101, is the obvious first day trip — a more upscale beach town with the famous Haystack Rock, tide pools rich with sea stars and puffins, and a small arts community. The two towns have a distinct contrast in character: Seaside is funfair, Cannon Beach is gallery. The Tillamook Head Trail between them is worth doing one way and catching a ride back.
Astoria, 17 miles north at the mouth of the Columbia River, is a working port city with a genuinely interesting history as the oldest American settlement west of the Rockies. The Astoria Column, a painted tower on the hilltop above town, gives a panoramic view of the Columbia, the Pacific, and the Coast Range. The Maritime Museum is one of the better museums on the Oregon coast.
Ecola State Park, immediately south of Cannon Beach, has some of the most spectacular coastal scenery in Oregon — old-growth forest above dramatic sea stacks, with views north to Seaside and south along the coast. Day use only; a parking fee applies.