This is a real gem in the National Park System crown. It is the location of the winter encampment of the Lewis and Clark expedition in 1805-06. Set in a beautiful pine forest near Astoria, Oregon, the site is home to a 1955 reconstruction of the actual fort, built according to a floor plan drawn by William Clark.
Despite nearly three months of constant rain, the Corps of Discovery ate well, and readied themselves for their return journey back east. Trails through the forest lead to a pretty little spring and to the landing on the Lewis and Clark River. Interpretive signs along the way, as well as beautiful exhibits in the visitor center really bring out what an amazing feat the expedition was.
My best tip for this site would be to not avoid it if it is raining. We were there on a rainy day, and it was just gorgeous. It also tended to add to the realism, considering the conditions under which the Corps lived here.