I haven't been to Vancouver Island, so . . . there's that. However, I have some ideas:
Are you taking the ferry from Seattle or from Port Angeles? Are you taking the car or leaving it wherever you board the ferry?
If you return to the US at Port Angeles, then you can explore the northern and eastern sides of Olympic (if you've done the WCT, then you probably don't need to drive the long distance to see the Hoh Valley or the ONP coast areas). You could backpack into one of the valleys (Dosewallips or Duckabush) or just day-hike from Hurricane Ridge.
Then ferry back to Seattle and spend one day in downtown: tour Pikes Place Market & around there. Might be a Mariners game to attend. Stay in a hotel or leave and drive to Leavenworth, WA.
Camp in NF campgrounds along Icicle Creek Road south of Leavenworth. Day-hike up to Lake Stuart & Colchuck Lake, staring up at Aasgard Pass into the Enchantments. Hang out in Bavarian-themed Leavenworth. From there, you could drive up to Lake Chelan and take the ferry to Stehekin and camp there. Or drive up to Glacier Peak Wilderness and hike into Spider Meadow or the Napeequa Valley.
On the way back towards Spokane, you could stop by Dry Falls State Park to see the cliffs that were once the world's largest waterfalls during the Missoula floods (Hey, gotta get some teaching moments in, too!). Then back to Spokane.