Is it possible gain muscle mass, while still keeping my body lean for this summer?

Weight Training: Choose 3-4 upper and lower body exercises in the gym, use weights that cause the muscle to fatigue in 30 repetitions. Combination exercise such as resistance rowing machines can help. Devote 30 minutes 3 times a week to muscle development and you should see results in 2 months. Maintain your good diet and aerobic habits to stay lean. Enjoy the change.