It depends on what your meaning of "alternatives" is. There is a wonderful ferry at Cathlamet. It would involve extra miles - but these are lovely miles as are the towns of Skamokawa and Cathlamet. The Cathlamet ferry is the last ferry remaining on the Lower Columbia.
Heading south - you would cut east on WA 4 just outside of Naselle - 24 miles to Skamokawa - then take the back road thru the Wildlife Refuge along the Columbia River. Cathlamet has an expansive view of the Columbia from its pier. From town you take the bridge over to Puget Island - fairly long bridge, but light traffic, then the ferry to Westport, OR. From Westport you can backtrack on US 30 to Astoria or head east east 10 miles to Clatskanie, south on OR 47 to Mist, then follow the Nehalem River to Nehalem on the coast (short 3-mile dirt section). This valley is one of the sweetest rides there is.
By doing this you miss Astoria, Seaside, and Cannon Beach plus a long stretch of US 101 - no great loss - but you also miss Fort Canby and the mouth of the Columbia plus the spectacular overlook at Arch Cape. If you're heading down the coast there will be plenty of other capes. Unless you really need supplies or a bike shop in Astoria, you can get all in Tillamook.
I've done both. US 101 north and south of Astoria can be extremely busy, but it has good shoulders. For my money, I'd do the longer, more scenic route using the ferry.