There is nothing wrong with doing some cardio (20-30 minutes maximum) after your weight training, but it should not be excessive or you will severely hamper the physiological and hormonal mechanisms that lead to muscular hypertrophy. Your best bet is to separate cardio and weight training by about 5 hours.
After an intense gym workout, your body is in severe need of nutrients, especially amino acids and carbohydrates. Your body needs this to replenish, and repair damaged muscle cells and feed the exhausted nervous system. Try to feed your body a high quality meal no longer than 15-20 minutes after completing your final set.
It is of vital importance to quickly consume the proper nutrients after a workout, and carbs play a very important role in this equation. Since the body is primed to absorb carbohydrates to refill muscle glycogen after lifting weights it is best to take in the “fast-acting” or “high GI” variety. Foods like white rice, white potatoes, rice cakes, and bagels will get into the bloodstream rapidly, which will elevate insulin levels and allow you to push nutrients into cells at a very quick rate.
Because whey protein is digested very rapidly it will supply your muscle cells with much-needed amino acids faster than all other protein sources. This is extremely important right after an intense workout; so failing to utilize the power of whey would most certainly be an error when the goal is more muscle.
The goal of the post-workout meal is to make sure that both carbohydrates and amino acids reach the bloodstream for distribution to muscle cells as rapidly as possible. Since fats tend to slow digestion it would be a big mistake to include a significant amount during the post-workout feeding.
While there is nothing more important than proper diet and training when it comes to making progress, smart supplementation certainly can hasten the process. So, if gaining muscle and losing fat are high on your priority list then take advantage of such proven supplements as creatine, glutamine, BCAA’s, and leucine by getting an efficacious dose of each either as soon as you are done training, or with the post workout meal.
In a perfect world all of us would be able to grab about an hour’s nap right after training to help our muscles and nervous system recover, but for most this is just not possible. I recommend that at the very least you avoid physically strenuous activity after working out. You should take at least a little time to just sit, relax and comfortably consume your post-workout meal. Weight training will only be effective if the body gets adequate rest to allow for the rebuilding process to take place.