Another drill we do from time to time instead of punching, we push each other, start with one shoulder, then the other, maybe one hip then the other. We learn to roll around the shove, instead of the typical rigid resistance. This is also the foundation for taking a hit. After you get the rolling around these things, even the punches, start replying back to the attacker with a punch or shove.
I don't know how this would fit into TKD, but they are good drills for being able to take a hit and be able to return fire. Also, it doesn't need to be done very often, just once in a while.