Can you reverse your plan? The coast is better viewed north to south. That way all the turn outs and sites are on your right. License fees will kill you once you find out how much they can be even for a day
Starting near Astoria, Ft Stevens state park is a good stsrting place. Lots of scenery, things to see and do. Your close to Ft Clatsup which is where Lewis and Clark wintered in 1805/6. Ft Stevens is a historic military fort thst guarded the mouth of the Columbia. It is also the only place on mainland US that was shelled during WWII. Gun placements that,you can tour. Further down the coast there is Beverley beach SP, but basically any of the state parks will offer great experiences. Be sure to firm up your plans and get reservations soon. It may already too late, but do it quick.
US97 in Central Oregon is a totally different experience. So plan accordingly.
Forget Portland, it is just another big city with lots and,lots of homeless living everywhere.