This is a simple consistency drill great for all but the youngest beginners. Players should be able to rally.
Challenge them to hit a serve, return and 2 additional rallies without an error. Do not play out the point, just these four shots. Put them on a timer and see how many times they can complete this successfully within the given time period. Rally must be quality shots from baseline - resist the urge to make it easy on them or let them take it easy on themselves :)
Adapted from "Live Ball: Let Them Play" by Oliver Stephens, TennisPro Magazine Sept/Oct 2013