Most people with an ICD can continue or return to the activities and hobbies they enjoyed before, including walking, cycling, swimming, golf, gardening, and many sports. The device is designed to allow your heart rate to rise normally during exercise without delivering unnecessary treatment.
Whether any specific activities need to be restricted depends on your underlying condition rather than the ICD itself. Your cardiologist will advise you on anything to avoid. If you are unsure about a specific activity, ask before resuming rather than avoiding everything out of caution.