I just read a bulletin from our health insurance provider about new evidence citing the detriments of sitting for most of the day. The article called sitting the new smoking, leading to all kinds of health issues, including diabetes, heart disease, joint stiffness, etc. 

We already know that physical activity, specifically, weight bearing exercise is important for maintaining and growing bone. What are some ways to work activity into your workday? 

Something I've tried at work is to sit on a stability ball for part of the day. It makes you more aware of your posture and helps activate your core muscles as you balance on the ball. You many need a large size to elevate yourself to the proper height for your computer.  Sitting on a ball won't lead to an increase in bone density, but it can help strengthen your core and encourage good posture, which will help you perform weight bearing exercise with good form.

Even better than a stability ball is an adjustable workstation that allows you to stand or sit.  Another strategy is to set reminders to get up every 30-60 minutes to stretch, walk down the hall, or even around the block if you can. Breaking up your sitting time helps prevent a slowdown in your metabolism that can lead to high blood sugar and other conditions. Try having work meetings standing up or walking. Drinking lots of water and green tea is healthy and will ensure that you need to get up from your desk more often. Even performing simple exercises at your desk can help activate your muscles and metabolism.  See the Resources page for more articles and ideas about sitting less and exercising more.

All you need for this one is a flight of stairs and a set of dumbbells! 
 I asked my chiropractor what exercises he would recommend that I do to build my overall strength and bone density (in addition to the biodensity machine that I work with once/week).  His suggestion:  stair climbing - adding weights as I practice and focusing on engaging the core muscles.

It only takes a few minutes to get your heart rate up and it's almost as demanding as running. You can do bicep curls with the weights while you ascend the stairs, then just hold them at your sides while you go down. 

Even if you only have 10 minutes to exercise, this one will make you feel like you have had a workout.  And there's no cost, no need to drive anywhere, and if you play music, the time passes quickly.

Finish up with a few stretches, like the leg swings as shown here: