Shadowing some DOs and getting a letter from one will definitely help your DO chances. It is one of the only real tangible things you can do to help stand out a tiny bit. Not to mention some schools require a DO letter. So thats definitely good to do.
Otherwise work - anything clinical or research will help your app. But even if you just end up just working any ole job its honestly fine. Nothing wrong with keeping up with some volunteering too; that stuff is good for the soul and good for the application. Just find a nice balance between some of that stuff and you will be good. The point is really to show that you are doing something with your time.