I'd highly recommend any of the coast state parks. They are awesome and dog-friendly beaches.
As stated, if you can, start Oregon from north at Astoria to south for better ocean views and better access to the large pullouts for viewing.
I'd suggest staying in three locations - one north, perhaps Fort Stevens State Park; one in the middle and one in the lower end. Then you won't have a lot of extra driving for siteseeing to cover the whole coast.
We've been there in May and had wonderful weather but as you know, you can't count on it. Bring warm layering clothes especially for beach walking. It's always cooler on the beach. Also bring a lightweight rain jacket.
Have fun planning!