Standing calf raises - stand anywhere on one foot with other leg slightly bent (touch counter or wall lightly for support if needed), rise up on toes and ball of foot, lower until your heel just barely touches the floor. You are lifting your body weight with your calf muscle. You will feel it pretty quickly. Do 10-15 reps, switch legs. Do 3 sets.
You can do this anywhere, while watching TV, while doing the dishes, while brushing your teeth, or get up from your desk/computer at work/home and quickly do them.