One that I like to use, is a scamble defence drill.

Basically you set up a 10m x 10m square (this can be altered dependand on numbers obvously), with each side of the square having different coloured cones.

You then have 3 players on each side of the square, who represent whatever colour the cones that they are on.

Then 3 players with bibs on in the middle. These are the defenders.

The coach shouts out a random colour, then those players on that named coloured cones have to get the ball to the opposite side of the square, whilst the guys in the middle have to react and respond. (kicking not allowed)

As soon as the ball is out of play, dropped, player is tackled, the coach shouts out another random colour.

The guys in the middle (the denfece), then have to react, regroup and respond to the next wave of attackers.

After a few minutes the guys in the middle are shattered and you swap with 3 fresh fellas.