Round Trip is my fave.
Two players begin across the net from each other at their service lines. They begin hitting volleys to each other, moving in with each shot. As they get to the net, they reverse and move back toward the service line with each shot.
Too hard? Have your students toss the ball rather than hit with the racquet.
Too easy? Specify which volley is to be hit (forehand/backhand) or alternate forehand/backhand volleys.