If you have never been to British Columbia, you may wish to bring your Passports, also.
You will miss lots of opportunities along the way by flying instead of driving. Think Yellowstone, Glacier NP, Idaho, etc. Oregon's 'driest' period in that area is October to April; July and August are the driest months in Western Washington state. I liked the month of MAY, Rhododendrons were in bloom! And it was still the 'off season' for room rates.
In no particular order:
http://traveloregon.com/trip-ideas/scenic-byways/Scenic Byways | State Of WA Tourism
https://stateofwatourism.comhttp://grb.smugmug.com/PHOTOS/2010/April/Silver-Falls-North/i-Fwwxf3FSilver Falls State Park. It's enormous and contains 10 gorgeous waterfalls. Trails galore too.
Salt Creek Falls ..... 20 miles
Soleduck Falls, Olympic NP, WA
Olympic National Park ,Rialto Beach, Ruby Beach
Snoqualmie Falls, Washington
Mt Rainier And Eunice Lake, WA
Alpenglow Wildflowers and Mt Baker From Skyline Divide, WA
Sunset Heceta Head Lighthouse, Oregon
Sunset Shi-Shi Beach, Olympic National Park, WA
Mt Shuksan Reflected In Picture Lake, WA
Latourell Falls, Columbia River Gorge, OR
Proxy Falls, Willamette National Forest, OR
Fairyland Falls, Columbia River Gorge, OR
Mt. Washington, Oregon,
Lewis River Falls, Gifford Pinchot National Forest, WA
Glacier Peak, Lenticular Clouds, And Moon, WA
Pre Dawn Light on Mt Baker and Ice Burg Lake, WA
Mt Adams Thru The Tatoosh Range, WA
Mt Baker and A Tarn, Park Butte, WA
Edith Creek and Mt Rainier, Mt Rainier NP
Twenty-Two Creek Waterfalls, Mt Baker-Snoqualmie NF, WA
Hall of Mosses Trail, Olympic NP, WA
Hidden Lake, North Cascades NP, WA
Mt Daniel and Robin Lake 2, Alpine Lakes Wilderness, WA
Rampart Ridge Alpine Lakes Wilderness, WA
Reflections Valley of the Ten Peaks, Banff NP, Canada
Lake Oesa, Yoho NP, Canada
Prusik Peak & Gnome Tarn, Enchantments, WA
Dragontail Peak Reflection in Tranquil Lake 2, Enchantments WA
Mt Assiniboine NP, Canada
Garabaldi Provencial Park, Canada
Eagle Cap Wilderness, OR
Baring Lake, Sawtooth NRA, ID
South Sister Reflected in Sparks Lake, OR
Sawtooth NRA, ID
Indian Henry's Hunting Ground, Mt Rainier NP, WA
Alaska Lakes Basin, WY
Mt St Helens and Spirit Lake, Mt St Helens NVM, WA.
Mt Adams From Norway Pass2 , Mt St Helens NVM, WA
Sauk Mountain State Park 2, WA
Goat Flats, Boulder River Wilderness, WA
Mt Adams and Tatoosh Range From Van Trump Park 2, Mt Rainier NP, WA
Sahale Glacier Over the North Cascades 2, North Cascades NP, WA
Desolation Peak 2, Ross Lake NRA, WA
Ross Lake and Hozomeen Peak, From Desalation Peak, Ross Lake NRA, WA
Doubtful Lake, North Cascades NP, WA
Steens Mountains, Steen Mountain National Recreation Lands, OR
Goat Lake and Foggy Peak 1, Henry M Jackson Wilderness, WA
Panorama Point 2, Mt Rainier NP
Silver Falls State Park, OR
Wahclella Falls 2, Columbia River Gorge, OR
Wahkeena Falls, Columbia River Gorge, OR
Metlako Falls, Eagle Creek Gorge, OR
Oneonta Gorge 2, Columbia River Gorge, OR
Clearwater Falls, Mt St Helens NVM, WA
North Cascades NP, WA
Ladder Creek Falls, North Cascades NP, WA
Boulder Falls, Boulder River Wilderness, WA
Tumalo Falls, Oregon
Wallace Falls State Park, WA
Wallace Falls State Park, WA
Lower Lewis River Falls 7, Gifford Pichot National Forest, WA
Sol Duc Falls, Olympic National Park, Washington State.
http://grb.smugmug.com/PHOTOS/2010/April/Silver-Falls-North/i-Fwwxf3FSilver Falls State Park. It's enormous and contains 10 gorgeous waterfalls. Trails galore too.
If you have never taken the ferry to Alaska... Will need a month (at least!), if you go there!