While you are in Phoenix -- the Musical Instrument Museum is surprisingly fascinating, and will have something for all ages; the Desert Botanic Gardens has great birdwatching if that's your thing; Taliesin West (architect Frank Lloyd Wright's establishment) can be skipped if you aren't interested in architecture - the Biltmore itself will give you a taste; the Hurd Art Museum.
If you weren't going in August I say, do hiking, lots of hiking - PHX has over 200 miles of walking/hiking trails, and climbing Squaw Peak (can't remember the new, PC, name-Pistewa?) and Camelback mountain provide a nice workout and wonderful views - try starting just at daybreak, take lots of water, sunscreen and hat and wear good hiking shoes/boots.
Main Street in Old Scottsdale is pleasant, a couple of the galleries actually interesting, and a fun old-fashioned ice cream parlor on the main drag.
But plan to get up early for the drive to Sedona, in order to spend the whole day there before going on to Flagstaff - again, there's great hiking, great vistas, and the town of Jerome is cute. It tends to be cooler than PHX, so that may give you reason to spend an extra day in the area.