Weight-bearing exercises are best for bones. Impact on bones help them adapt and become stronger. Weight-bearing activities can be either body weight from running or gym weights. Activities can be either low impact and high impact.
Low impact activities are less stressful on joints and already fragile bones but they still place more stress than non-weight bearing activities. A good rule of thumb for low impact activities is any activity that requires one foot on the ground at any given time. Low impact activities include walking, step-aerobics, elliptical machines, step-climbing, elastic-band resistance training, weight-lifting, etc.
Low impact activities can help reduce bone loss.
High-impact activities can be the most beneficial for people who want to build bone density and who are not at risk of fracture. However, participate in these activities sparingly to avoid overuse injuries of ligaments, tendons, and bones. It is important to incorporate active rest days in between high-impact activities. Active rest days should still involve other low-impact activities to let your body recover and to reap the benefits of a consistent exercise program.