Circuit Training can improve strength, aerobic fitness, and flexibility all in one workout. The idea is to workout continuously hopping from one exercise to the next.

Example:
40 pushups
1 minute of jumping jacks
35 sittups
10 pull-ups
30 seconds of squat jumps
30 seconds of rowing machine
1 minute of front kicks
1 minute of shadow boxing
lay on your back and raise your heals a foot off the ground for 20 seconds, etc.

You can develop a routine or do a new routine every time you workout.The possibilities are endless, making this form of exercise ideal if you get bored of routines. Here are examples to choose from. Remember, you are not limited to these. Get creative. If you want to throw snowballs for 1 minute, go ahead. Do anything, as long as it keeps your heart rate elevated. This is the secret to gaining aerobic fitness while strength training.

The crazy circuit training list
First of all you can fill your entire circuit with exercises from our strength training section. As long as you go from each exercise immediately to the next you can sustain an increased heart rate.
Bicep Curls
Squats
Squat jumps
Tricep extensions
Lat pull-downs
push-ups
pull-ups
sit-ups
jumping jacks
clean & press
shadow boxing
front kicks
side kicks
back kicks
forward punches holding light dumbells
fast dancing
sprinting
jogging
walking
Cardio-machines
stairmaster
treadmill
stationary bike
medicine ball (pass it back and forth with a partner)
trunk twists holding a broom handle across your shoulders
swimming laps (Do the other exercises on an exercise mat on the pool deck. You can use the high dive for pull-ups.)