First aerobic period: 5 minutes brisk walk or slow jog Pit stop: Walking lunges. Step forward, extending your right leg in front of you. Bend your knees, and lower your body toward the floor between the back and front legs. The back leg will bend and the back heel will come off the floor. The front leg will be bent to a right angle, with the knee never going in front of the ankle, remaining directly in line with the ankle. Come up and switch legs, bringing the left leg forward, and repeat. Do 10 "switches."
Second aerobic period: 5 minutes brisk walk or jog Pit stop: Jumps. Take a stance as if you were about to dive into a pool. Crouch down slightly, put both hands behind you, and then, in this "half-squat" position, "dive" forward as far as you can by pushing off on your feet and thrusting your arms forward. Try to land with a soft impact by bending your knees. Then immediately take two small jumps back. Repeat the sequence 6 times.
Third aerobic period: 5 minutes brisk walk or jog Pit stop: Push-ups. You can do them any of a number of ways, depending on your fitness level -- the "assisted" version on your knees or the regular one using the whole body weight. Beginners can keep the feet on the ground and push up from the seat part of a park bench, keeping the body at an angle to the ground. The steeper the angle, the easier it is; you can make it even easier by standing a couple of feet away from a tree, leaning your upper body against it and pushing off. Return the upper body till it's touching the tree, and repeat. Reps: as many as you can.
Fourth aerobic period: 5 minutes brisk walk or jog Pit stop: Tricep dips. Sit on a park bench, legs extended, feet on the ground, hands at your sides grasping the edge of the bench, palms facing behind you. Slide your butt off the bench and lower your body towards the ground by bending the elbows. Stop lowering your body when your shoulders get just above your elbows, and then bring yourself up again by straightening the arms. Keep your back perpendicular to the ground. The closer you bring your feet in (and the more the knees are bent) the easier the exercise. Ten reps.
Fifth aerobic period: 5 minutes brisk walk or jog, slowing down the last couple of minutes to bring the heart rate down.
Finish up with: A set of abdominal curls and some light stretching, and walk home.
Exercising outdoors with a workout like this not only lets you train both anaerobic and aerobic energy systems, it gives you the added benefit of exposure to sunlight. That produces vitamin D in the body, vitally important for bone health, and has the added advantage of raising serotonin levels (nature's own "natural" Prozac.).