Here's a core workout program, takes about 30 minutes. You can do these in the living room, don't need weights. Just need to be disciplined about doing them. If you do them right, and you do these consistently, you'll be in pretty awesome shape. The cliff notes version i.e. if you have only have time to do some of the workout, just do the squats or lunges and then sit-ups and push-ups.
Core workouts target the torso, or stomach, back, hip/upper thigh area. This is where all the power is in your body.
I quit my gym membership a long time ago because I've been swimming at the local pool and running or biking outside. And then doing these core workouts at home makes that $80 gym membership pretty pointless.