I am not sure if they still rent any bikes but you might want to try Pinnacle Peak Cyclery or Bicycle Ranch. They are both in North Scotttsdale and you would be out of the main city part shortly or right away. You might also try Bicycle Showcase and Landis which are close to downtown Scottsdale. It isn't flat up in that area but it isn't real hilly either unless you choose to go down to Bartlet lake. There are lots of good long roads and you can get away from the traffic.
http://www.pinnaclepeakcyclery.com/ Their website says they do road rentals
http://www.bicycleranch.com/
http://www.bicycleshowcase.com/
http://landiscyclery.com/site/intro.cfm