You can easily create numerous fat-burning workouts that you can do in 15 minutes or less. Here are a couple:
1. Double-Doozie Circuit: set up a circuit of 4-5 compound exercises. For example, squats, rows, push-ups, and clean & presses. Have a list of 4 cardio activities also. Such as mountain climbers, burpees, high knee runs, and squat jacks. Perform the first exercise (squats in this case) for 45 seconds. Then immediately perform the mountain climbers for 30 seconds. Repeat. Rest for 30-45 seconds. Move to the rows. Perform rows for 45 seconds, then perform the burpees for 30 seconds. Repeat. Move to the push-ups with the high knee runs. Then the clean & press and squat jacks. The whole workout should only take about 10 minutes.
Use challenging weights on the strength exercises. Warm-up prior to starting any workout.
2. Do It or Die: Choose 6 exercises. Strength and cardio activities. For example, weighted forward lunges, jump rope, squat jumps, clean & presses, plyometric lunges, and burpees with push-ups. Grab a die. Number the exercises 1-6. Roll the die. Do the exercise for 30-45 seconds. Roll the die. Do the exercise for 30-45 seconds. Roll the die. Do the exercise for 30-45 seconds. After 3 rolls, take a 30-45 second break. Repeat for 15 minutes.
I hope this gives you some good idea. Most importantly, have fun with it! Fitness doesn't have to be boring and a chore!