I've done a ton of weight training, boxing moves, polys and mma stuff but I recently babysat my niece and nephew which proved to be the most effective training ever.
I hung one off each arm and started shadow boxing. Not only did they start swinging around on my arms but the little rat bastards also kept trying to kick me which meant I had to move my body around whilst simulating punches.
Needless to say we did that on and off for 4 hours and I was completely fried.
I strongly recommend small children to develop punching power and increase the bob and weave whilst counter punching skills.