The most common aerobic activity is brisk walking, which is perfectly fine, as long as you walk like you’re late for a train rather than window shopping. For the aerobics to have their best effect, you really need to work up a sweat. (Yes, it is ladylike to sweat, because it makes you a healthier woman.)
Jogging, swimming, bicycling, working out on a machine such as a cross-country ski machine, stair stepper or elliptical trainer, dancing (but not to slow dances like the fox trot), aerobics classes… All work great! Just note that if you want to improve the state of your bones as you improve the state of your heart, you have to be standing as you exercise.
Bicycling and swimming are great for the heart but will not make the bones denser. Some women prefer cross training: walking or jogging some days and swimming on others. That allows for great full-body workouts. Swimming, for instance, works the muscles in the arms in a way that walking does not.
You always say you're going to do 200 crunches every time The Bachelor comes on, but you barely make it through 20 before succumbing to lounging on the floor. These made-for-multitasking routines will keep you going until the credits roll.