Upper-body circuit (weight) training -- move from exercise to exercise without a minimum amount of rest in between exercises. I'd avoid any cardio that involves your knees until you're sure you're completely healed from your surgery -- you can injure joints just as easily with "low-impact" exercises if your form isn't right.

Okay, I'm seriously becoming less and less willing to do the elliptical after my 40 minute weight routine, so this suggestion intrigues me. But I have to ask -- what exercises are you doing for your upper body, that you could keep going for 30 minutes (i.e., the suggested amount of time we should spend doing cardio)? I mean, don't you run into the problem of hitting the same muscle groups with diminishing returns? For example: I can't do the pullover, pectoral fly, shoulder press and chest press during the same workout and expect to see good results during the latter half of the workout, because I'd be hitting some of the same muscles again and again. I'm not sure that would even be a wise use of my time in the weight room, actually.

If you've got an upper-body routine worked out that avoids this problem, please share it! I would love to sub out the elliptical in exchange for more weights work, and I'm in love with the idea of really shaping my arms.