Never underestimate the power behind a punch. Traditional boxing and kickboxing classes -- you know, the ones that use actual boxing gloves and heavy bags -- do wonders for enhancing strength and total-body conditioning. According to most calorie-burn calculators, a 150lb person can expect to burn roughly 300 calories in a 30-minute workout (about the same as jogging at a moderate pace), but boxing requires total body engagement, as well as lateral and rotational movements as you slide, punch, duck, and twist through a standard routine.
These multi-planar movements fire up your core, while punching a heavy bag helps develop serious upper-body strength. Since most boxing classes also incorporate bodyweight exercises, such as squats, lunges, and push-ups, you're essentially checking all the boxes on the list of "must-do's" when it comes to leaning out fast.
The "Big 4" compound exercises
If I could only choose four exercises to prescribe to anyone seeking to shape up in a hurry, I'd point to squats, lunges, pull-ups, and push-ups. The reasoning is simple: These exercises can be modified for practically any fitness level, they can be done with or without added resistance, they target all the major muscle groups, and they're reasonably accessible with or without a gym membership. Plus, there are about a million variations you can incorporate to help keep your workouts interesting.
Two quick words of advice, though: First, focus on form. If you fail to perform a compound exercise correctly, you open yourself to the potential for injury. And second, work yourself to fatigue. You can do this by adding weight to the exercise or by performing more repetitions per set. Either way, the last one or two reps you perform should push you to your limits. It's when you live at the edge of your capability that real change takes place.