The Centennial Trail ride is pretty boring in some places, and all around pretty flat, but the river scenery is really nice. In the Spokane Valley, it gets shady and goes through some pretty (and green) areas. I admit, I haven't done the whole thing, but I've done the downtown Spokane segment, and everything from the Spokane Valley to Coeur d'Alene's Northwest Blvd trailhead, then from the resort to the very end. I guess I'm probably missing about 10-12 miles of total scenery I think where you start out in CdA you'll be missing the lake scenery, but you will see everything along the river.

Hayden, Newman, Avondale, Hauser... all pretty territory, and I'd imagine quite significantly more hilly than the trail. I guess it depends on whether you want to take day 2 easy or not