Weight lifting. But if you want to do it without lifting weights try HIIT (High Intensity Interval Training) exercises. These workouts force you to work hard in short spurts with little recovery time. The result is a spike in your metabolism creating greater after burn (the amount of calories you burn after your workout is over).
HIIT exercises are great because you get the combo of strength training an cardio in one workout.
I built my HIIT exercises (20x10, Sexy Booty, Core Blast) from workouts I found here and at ab-workouts-guide.com If you like em try Cardio Fight Club HIIT, Strong Inner Warrior, Olympic "Circuit Challenge" they're fun and challenging.
Good luck and God Bless!