There are so many equally beautiful places to hike in the Washington and Oregon Cascades that I wouldn't worry too much about missing the Wonderland. Our National Forest Wilderness Areas are just as beautiful as and far less crowded than our National Parks. Except for a very few highly impacted areas, they also involve far less bureaucracy--you can generally hike and camp where you chose.
A very few suggestions--there are many, many more!:
Washington: Spider Meadow/Lyman Lakes/Image Lake/Buck Creek Pass Loop, Glacier Peak Wilderness. Hoh River/High Divide Traverse (can also be done as a partial "lollipop" loop), Olympic National Park (this one is in a national park, and does require backcountry camping permits). Chelan Summit Trail, Chelan/Sawtooth Wilderness.
Oregon: Three Sisters Wilderness Loop. Or, if you don't mind a long drive to get there, anywhere in the Wallowa Mountains of northeastern Oregon.