This workout is really simple, yet really effective in working on reacting, explosiveness, and speed with punching.
Drill #1
Jab, Cross, Jab, Cross
Practice this drill for 5 minute increments.  Have your pad holder move around and flash the pads.  Attacker should be quick and close.  Pad holder – throw in some hits to add blocking to the routine.
Drill #2
Hook, Cross, Hook, Cross
Practice this drill for 5 minute increments.  Pad holder flashes the pad randomly and the attacker hits as fast as he can.  Mix with some uppercuts at the last technique and also work blocks if you feel.
Use your favorite side, be that your strong or weak side.
If you have time, work on the other side.