Split your forwards and backs. Your playing area is between try-line and 22 on the same side of the field, touch-lines apply as touchlines.
You need a ball and a stopwatch.
The backs attack the try-line from behind the 22, using full width of the field. Forwards defend the line; they touch the opponent carrying the ball with two hands instead of tackling him or you can have them touching opponent with one hand below the waist (probably better).
You blow the whistle to start, from which time the attackers have one minute to score as many tries as possible (aim at fewest six tries in a minute). They score by crossing the tryline and then the 22 alternately - so, if you score over the tryline, you turn immediately and launch an attack at the 22. There are no stoppages. If the attacking side passes forwards, drops the ball, knocks-on, the game reverses direction immediately but the same side keeps the ball.
At first you will be shocked and perhaps disappointed at how poorly the ball-carrying side performs but the improvement is rapid. After the backs have tried to score as many tries as possible in one minute, give the ball to the forwards - they do surprisingly well compared to the backs - initially, anyway.
Between sessions, talk about defence lines for the defenders, attack lines for ball-carriers, support, depth, running lines. You will be astonished how much you can teach through this game.
