Cycling and swimming are not without their benefits, but if you want to increase your bone density, orthopedic surgeon Dr. Vonda Wright says you need to "bash your bones." In other words, strengthening your muscles simply isn't enough. You need to expose your body to "dynamic impact," i.e. the kind of exertion from walking uphill, jumping rope, climbing stairs, or weight training. With regard to weight training in particular, the exercises that give the most benefit to one's bones are those that mimic everyday movements. That means using one's body weight with free weights is far preferable to machines like the leg press.
Hmm... I wonder what other dynamic impact exercises exist that are like climbing stairs?
Thanks to Pat Stanley for the link.