I'm a bit biased, being based in the Portland area but I'd recommend focusing your time/efforts in Oregon.
Springtime in the Columbia River Gorge is exceptional, lush greens and good water flow in the waterfalls. Central Oregon is not too far, check out the Painted Hills in John Day, OR. If you want to photograph seascapes, the Oregon coast is also very good. Canon Beach/Ecola is sandy beaches with sea stack rocks. As you go farther south you get more rocky/rugged seascapes.
Washington is also beautiful, especially the Cascades around Mt Rainer and north cascades up to Mt Baker/Mt Shuksan. You're a bit early for wildflowers and mountain scenes though, that's more July (possibly June this year). The Olympic coast is great, but a bit remote and a long drive from Seattle, not quite as accessible and fun with associated towns/attractions as the Oregon coast can be.