No WORK! NO talking about work, NO checking on work, NO working on work.
In order to give your mind and body a true vacation, regardless of how short, from work you must not only detach but empty your mind of anything work related.
I know it's hard. You were hoping to do a few things, catch up on email or even jot down some ideas for the new project; Well, NO. There is no way you can go back to work refreshed, ready to take on the "next thing" without giving yourself some mental Time OFF.