Aerobics:

Calories per 10 min : Moderate-50; Moderate high-75;
High - 110.
Benefits : Can be done anywhere. Good for body composition and cardiovascular fitness. 
Considerations : High impact aerobics may cause leg injuries. Low impact aerobics may not raise heart rate. Difficult to measure week-to-week progress.

Cycling
Calories per 10 min : 5.5 mph-40; 10mph-70; 13 mph-110;
18 mph-145
Benefits : Strengthens leg muscles. Good for body composition and cardiovascular fitness.
Considerations : Cycling in traffic has risks. Good bicycles are expensive. Most people pedal too slowly to derive good benefit.

Climbing stairs
Calories per 10 min : 7" riser:30 steps/min 1 at a time : 110 40steps/min 1 at a time : 140 ; 40 steps/min 2 at a time : 190
Benefits : Good for body composition and cardiovascular fitness.
Considerations : Too strenuous for some people. Very boring.

Yoga/stretching
Calories per 10 min : 40 or less
Benefits : Excellent for improving flexibility. No equipment required.
Considerations : No improvement in cardiovascular fitness, reduce body fat or improve muscle endurance and strength.

Walking
Calories per 10 min : 3 mph - 50; 4 mph - 60; 5mph - 95; 5.5 mph - 110;
Benefits: Good for body composition and cardiovascular fitness. Low injury rate and very safe.
Considerations: Takes longer than other activities to improve cardiovascular fitness and derive the full benefit. Does little to condition muscles of upper body and waist.

* Approximate calorie usage for a person weighing 150 pounds. Add 10% for every 15 pounds over this weight and subtract 10% for every 15 pounds under 150 pounds.