There are 28 places on the National Register of Historic Places in Olympic National Park. We start the list with the first 6 (alphabetically):
Altair Campground Community Kitchen - Campground central kitchen built by the Civilian Conservation Corps.
Botten Cabin - The Botten Cabin, also known as the Wilder Patrol Cabin, was built in 1929 in the Elwha River valley for Henry H. Botten
Canyon Creek Shelter - It is the last remaining trail shelter built in the park by the Civilian Conservation Corps from Camp Elwha. The shelter was built in 1939, shortly after Olympic National Park was established from the U.S. Forest Service-administered Mount Olympus National Monument.
Coastie Head Cabin - unable to find any info about this, but I assume its something that the Coast Guard used.
Dodger Point Fire Lookout - The Dodger Point Fire Lookout was built in 1933 in Olympic National Park as a fire observation station. The single-story frame structure is located on the peak of Dodger Point above the timber line at an elevation of 5,753 feet (1,754 m).
Eagle Ranger Station - The Eagle Ranger Station, also known as the Eagle Guard Station and presently known as the Sol Duc Ranger Station, is a complex of three buildings built in the 1930s in what would become Olympic National Park. The primary structures were built by the U.S. Forest Service in what was at the time the Olympic National Forest., While the main residence was built by the USFS, the generating plant and landscaping were built by the National Park Service using labor provided by the Civilian Conservation Corps.