ou'd be hard-pressed to track down Kostas Argiropoulos's boxing gym, Boxer's Workout, without a map. It's in the two-car garage beside his house on a residential side street. You'll know you've found it when you see the sign (painted by an artist who trains there) that proclaims, "Beginner's Welcome!" He may have boxed competitively, but Argiropoulos makes his students' workouts spirited and fun (and, on the first visit, free). Which is not to say they're easy. Your core will be on fire, and your arms and quads will be numb when he's done with you. It's as intense a workout as they get. But we bet you'll come back. Nesin