Each person has a Resting Metabolic Rate (RMR), which is an estimate of how many calories you would burn if you did nothing but rest for 24 hours. As you perform activities throughout the day, more calories are needed. To lose weight without dieting, you need to increase your activity level. Some of the things you can do include:
- cardio training
- weight lifting
That's just a very small list of the variety of activities you can do to increase your calorie expenditure. By increasing you daily calories burned, and keeping your calorie intake the same, you will be able to create a calorie deficit and achieve weight loss. The ultimate goal is to create a calorie deficit, and you have just achieved this by increasing your physical activity levels.
One of the best ways to increase your metabolism and burn more calories is weight lifting. Lifting weights will stimulate lean muscle growth, and each pound of lean muscle you gain will help you burn an extra 50 calories per day, and that doesn't even involve you doing anything. If you were to just put on five pounds lean muscle, that would translate to an extra 250 calories burned every day - this is roughly equivalent to running (at 6mph) for about 23 minutes.
The more lean muscle you gain, the easier it becomes to burn off fat. On top of this, if you were to lift heavy, and have high intensity workouts, you will increase your metabolism throughout the day, leading to more calories burned. Studies have found that intense exercises stimulate both energy expenditure and lipid oxidation for prolonged periods after completing the workout. One study found that "resting energy expenditure measured the next morning was significantly higher (+4.7%) than that measured without previous exercise". 
As you can see, the benefits of intense exercise are many. You burn calories during the exercise, you have higher resting energy expenditure for long periods after the exercise, and you build lean muscle mass over time, which leads to increased energy expenditure. How's that for losing weight without dieting? Of course, if you did combine it with a sensible diet, you'd get there even faster.