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: