Any aerobic exercise is good - as long as it is something which would not allow you to fall - bicycling become a little for dangerous, for example. Swimming is fantastic, for example - and even just walking in the pool helps elevate the baby weight and help the back. Some OB's worry about getting in the pool during the last month of pregnancy, so check with yours before your ninth month. A stationary bike, an elliptical, other trainers like this would also be helpful.
Stair master will definatly help you. 10-15 min a day 3-4 times a week.
http://www.livestrong.com/article/507971-how-to-keep-the-weight-down-while-im-pregnant/