Mowing the lawn should be one of the activities choices for exercise.  Pushing a mower is almost like using a sled with weights one it while running.  The weight and model of mower should be a option to get the best results in the workout.  Plus what the weather is for that day you are doing the mowing play a factor in that workout.  Personally on days when it above 90 degrees Fahrenheit is much more strenuous than days it's below 90 degrees.  Just an idea I've had for a while.