A great exercise for your calves. Using only your body weight and a step and can be done almost anywhere.
Stand on the top of the step (on just the balls of your feet). Try your best to do this without holding onto anything unless you lose your balance. Once corrected, give another try of doing this freehanded. Dip your heels to the ground (keeping your body straight) then step up onto your tiptoes. Repeat. To alternate, do one side first, then move to the other.