If you don't mind getting to Idaho late, head up towards Grand Coulee for some sightseeing and hiking.

I'd definitely suggest staying in Coeur d'Alene, if you can get a place on the riverfront. It's much nicer, IMO, than Spokane.

If you're taking the long way heading up to Hwy 2 via Sedro-Wooley, stop at the LaConner tulip festival and Snow Goose Market for a giant ice cream. This might throw off your schedule significantly, so plan for a stop somewhere in WA rather than the first night in Idaho.