Here's another singles activity for two or more players from Aaron Fox's presentation at the 2012 PTR Symposium. It has the added bonus of also being a fitness challenge. But I guess all singles drills are a fitness challenge, aren't they??
Two players (or teams of players) begin on each end of the court diagonally from each other at the intersection of the singles line and the baseline. One end or team is designated as the Running Team. Coach feeds first ball to the Running Team to opposite half of the court from wherever they are lined up. So for example if the players are lined up on the deuce side corner, coach feeds first ball into the middle of the ad side of the Running court. Players play out the court. Player who wins the point runs back and touches the corner where they began the point, and coach feeds next ball. Coach always feeds Running Team side regardless of who wins each point. First team to 11 wins and teams switch roles (non-running team is now Running Team for round 2).
Easier - use slower balls and smaller courts.
Harder - put restrictions on play such as hitting balls beyond service line, only forehands, only backhands, only cross court, etc.
Aaron, I hope you don't mind if I call this Carolina Corners when I use it. :)
UPDATE: My orange ball students love this game. I have been using this frequently, pairing it with whatever stroke we are emphasizing during the lesson. For example if we are working on our forehands, I play this game by making the first fed ball to their forehand.