Techniques of simulating a fistfight
For example, to fake a right-handed punch, hold your left fist near your chest and as you throw the punch (missing your partner's nose, hopefully), thump your chest with the other hand to produce the expected sound. Eyes will be on your punching fist, so if your partner fakes the "impact" well no one will see your left hand make the sound.
A knee to the head makes a great finale. Take your partner by the shoulders (or ears) and bring their head down as your bring your knee up. Again, at the moment of impact slide your hands down to slap your knee while your partner fakes the recoil. An artfully concealed gore packet completes the effect.