Low Impact/High Calorie Burns

Hello - I've been on mfp for a while but have not posted much. I'm in need of some Low Impact/High Calorie workouts. My rec center gym has been taken over by people with New Year's resolutions, and they haven't seemed to drop out yet I was doing well on the elliptical for 35-45 minutes + 15-20 minutes on the weight machines. Yes, not hard core, but it was a routine that I could fit in 4x a week that was working for me. I had to stop running and stop the stair master because of my knees. I'm still doing PT, but I don't think running will ever be back in the cards. The problem is the elliptical and cross training machines and rowing machine are always taken. So all that's left is stationary bikes. And the weight machines are also being hogged by people hanging out. Rather than hoping that these people will quit soon, can anyone point me in the direction of low impact, but high calorie circuits or sources? I can do the bike at a high pace for 40 minutes, but I don't have any more time that I am willing to devote - I would really like to burn more calories than 150-200 calories per workout. I'm also "scared" of doing free weights because I'm worried my form will be all screwed up - I feel like a newbie again