Shadowboxing is the most underrated form of training
1. It requires no equipment.
2. It simulates a fight if you use visualisation techniques.
3. It is superior for developing speed compared to a heavy bag.
4. You can practice every punch perfectly, whereas on mitts/bag you have to adjust your punch unnaturally to hit a false target.
5. It trains your footwork if you do it properly; this is hard on a heavy bag as the focus is on punching all the time.