A couple of suggestions that may save you some time money and aggavation. You do not "need" a rental

In driving from Vancouver to the OP, make your ferry reservations early and leave your hotel in time to make the ferry in case you run into one of the frequent delays at the border. (If you arrive on Whidbey national rental car used sales Island early, you can check out Deception Pass, Penn Cove, Captain Whidbey Inn, and the cute little town of Coupevill.)
A couple of suggestions that may save you some time money and aggavation. You do not "need" a rental car in Seattle when you arrive to see anything within range of the bus system which is broad. Take the Amtrak bus or train to Vancouver. Rent a car in Vancouver to see the things outside the city that you want to see. Many things including Stanley Park are very walkable.
Sunday-Week 2 cross the bridge at Longview from I-5 and head west to Astoria o US 30. Take US 101 south at least as far as Cannon Beach to see Haystack Rock. You may decide you like the Oregon coastline more than the Washington coastline.
To get from Hood River there is a good paved Forest Service road that goes from Carson WA (Rt. 14) all the way to Randle on US 2. This will take you up on the east side of Mt. St. Hellens. There is a side road that goes over to the Windy Ridge Viewpoint off FS Rd 25. This will show you some better scenery than the I-5 route. You can enter Mt. Rainier on WA route 123 off US 2 a few miles east of Packwood.