You could do a version of the ACA's Washington Parks Route -- it goes through North Cascades National Park, Mt. Rainier National Park, and Olympic National Park.
It starts/ends in Sedro Woolley, which is a day's ride from Vancouver. If you throw in some time hanging out in the San Juan Islands, you could easily fill your time. There a bunch of ways to then end your tour in Vancouver -- like ferrying over from the San Juan Islands to Victoria, then to Vancouver....or just ride north from Sedro Woolley....or ferry from Port Angeles (on the Olympic Peninsula) to Victoria -- then to Vancouver.