I would not recommend an online course. You cannot get the feel or timing as well without a good instructor.
While we may only need the Adult CPR/AED training to maintain our certs, I recommend everyone take the full CPR/AED/First Aid course for Adult/Child/Infant. These are skills that could save a life anywhere anytime. I am a Red Cross instructor and have taken the American Heart classes as well. Either is fine and both will prepare you for almost any emergency.
But we should all at least mentally go through the steps for performing CPR/AED/First Aid from time to time. I review my manuals every two months unless I teach a CPR class within that time. (I review and prepare for each class unless I just taught a class no more than two weeks prior to my next class.) You don’t need a manikin to practice. A firm bosu works great for compression and you can simulate breaths without the manikin. Just rereading the manual is better than nothing.