Mostly volley drills.
One - start playing half court tennis. Play points where the ball has to land in the service box. If it lands out of the service box it's out. When you start getting too good at that with just one box, play half court using both boxes.
Two - I've used this before. Two players at the net, volleying. Object keep the ball down. If the ball gets up drill someone...safely.
Three - I did a session with a friend of mine with Richie Reneberg's coach and he did what he called a monster drill. Basically, the coach fed a ball and we went after it. The catch; you can't let the ball bounce. And the coach would hit it anywhere. Start with some volleys, and the start throwing in lobs wide shots midcourt shots, and even balls out of the court. Make the net guy run and chase balls untill he can't chase any more. If he gets too good at retrieving, put 5 or 10 pount weights on his ankles and go again.