Hi.
This is a fairly common problem for people with large breasts. The variety of cardio exercises might be less of an issue for you since you can integrate walking, bike riding and the like, which do not cause as much jostling as running or jumping jacks do.
Sit-ups are more difficult because they often involve compressing the chest area. However, some sort of abdominal exercises are important for those with large breasts because they actually help strengthen the muscles in your lower back that balance out the weight of your breasts.
Try crunches, modified sit-ups that involve putting your hands behind your head and bringing your upper back up only about 6 inches or so off the floor. These tend to be better for the back than full sit-ups anyway because they more accurately isolate the muscles in your abdomen. You may even try oblique crunches by bringing your elbows up in sequence toward the opposite side of your body, when you raise your torso.
I don't know if you have access to gym equipment, but some gyms have ab crunch boards or machines (which are weight bearing) that support your feet and allow you to do crunches more easily and at various angles to intensify the results of the maneuver. If you do go to the gym, ask the attendant to point out these machines the next time you're there.
I hope this helps!
Take care.
