Here is a workout that I designed and use myself when my schedule is hectic. If you can grab 20 minutes out of your day three times a week, then this might be worth a try. You will need one set of dumbbells weighing 5-15 pounds each. The exercises require that you be able to perform 20 repetitions without stopping, so choose lighter weights than you might use when performing 10-12 reps per set. (*As always, consult your doctor prior to starting any exercise program.*)
The Warmup
20 jumping jacks 20 knee raises (like marching in place, 20 on each leg) 20 jumping jacks
20 tricep kickbacks 20 toe touches (lying down w/ feet in the air, holding one dumbbell w/both hands) 20 lunges (10 on each leg holding your weights) 20 reverse crunches
20 second break
20 shoulder presses (standing) 20 count plank (get in pushup position resting on elbows and count to twenty) 20 pushups 20 count plank
The Cooldown
Stretch every muscle group while focusing on deep breathing. Drink at least 2 glasses of water.