Not too far away from Downtown Scottsdale, you'll find GR Herberger Park. This is a great location to play tennis, with four public and free tennis courts, all lighted. Just be wary of choosing the west most tennis court, as one of the lights is out as of 11/20/17. The tennis courts themselves are serviceable, but the lines are faded and courts themselves are pretty dirty. If you're breaking out some new tennis balls, just keep that in mind. Other than those minor issues, the nets are great and there are no cracks in the courts which can be an issue at older facilities.
The courts are definitely popular - I went at 7:30pm on a Monday and all the courts were taken. You'll see some group lessons and other friends playing, so snag a court as soon as you can.
The park also has two sand volleyball courts and plenty of open space. Can't go wrong coming here.