i am currently lifting 3x a week, at 6:30am. lifting in the morning is the only way for me due to massive work schedule.

whats the best way to do cardio w/ this schedule? 3x a week on off days in the morning/empty stomach? low intensity, high intensity; for how long? and what method works best (i.e.- running, bike, elliptical, treadmill, jujmprope, etc)?