There were forts already in place at the mouth of the Puget Sound in the Pacific Northwest. If you get a chance to get out that way the forts are definitely worth checking out. The Oregon/Washington coastline would have been nearly impossible for the Japanese to invade IMO. High cliffs make up most of the coastline. And the distance they would've had to travel would have curtailed any sort of surprise attack.
'Eight coastal military forts, most of them dating from the turn of the 20th century, now serve as historic state parks. Fort Columbia and Fort Worden survive almost fully intact, but all - Fort Casey, Fort Flagler, Fort Ebey, Fort Ward, Manchester and Fort Canby at Cape Disappointment - have at least some original structures and wonderful water views.'