Here's a fun variation on the 'tennis ball sandwich' ball pickup game which turns it into a fun team relay. Best for large groups.
Divide players into teams at one baseline. Players layer balls between racquets, starting with a minimum of three racquets (two balls). They move together to the net and back without spilling. If they succeed, they add another racquet and ball to their stack. If they fail, they return to the baseline and try again. First team to move successfully up and back with the highest 'skyscraper' wins.
Adapted from The Tennis Drill Book by Tina Hoskins.