Biking
For a 145-pound woman, bicycling at a reasonable 12- to 14-mph pace burns about 560 calories per hour. But if you up the intensity to 16 mph, cycling can burn as many as 835 calories in an hour. Try to pedal rather than coast. You might also want to try interval training. When the bike path is clear of other cyclists, sprint for a couple of minutes, slow down to your normal pace until you feel rested, then push hard again.
If you like exercising with a partner, tandem cycling might be the way to go. Unlike other activities (like running) where twosomes of different levels can slow one person down, doubling up on a bike is a breeze. The stronger rider sits up front and does all the shifting, steering, braking, and heavy pedaling; the weaker cyclist rides in back and kicks in extra power. Take the effort level to a moderate intensity and you'll both burn about 500 calories an hour. We guarantee you'll pick up the rhythm instantly--even if the last bike you rode had a banana seat.

Inline skating
For a 145-pound woman, inline skating burns approximately 500 calories per hour. To boost your calorie burn on Rollerblades, skate as continuously as possible, minimizing the time you spend gliding. You might also try interval training. When the path is clear of other skaters, sprint for a couple of minutes, slow down to your normal pace until you feel rested, then push hard again.

Swimming
Whether you're training for your first triathlon or burned out on cardio machines, swimming is an excellent head-to-toe workout (and it burns 700 calories an hour!). Here's how to get started:
Find a pool Try a community center, YMCA, health club, or even a local community college. Many offer weekly times when anyone can swim.
Start small Do two full laps (back and forth equals one), pause to catch your breath, and repeat three times. Try to practice two or three times a week.
Perfect your form Use every other lap to do a different drill: Hold a kickboard to concentrate on your kicking, or swim with a buoy between your legs to work on your stroke.
Build it up When swimming 300 yards feels easy, increase your total distance by up to 10 percent per week. Join a masters team for built-in guidance and motivation (find one at usms.org).