I'm about to tell my advisor I want to leave my PhD. Do I mention this when scheduling a meeting or do I drop a bombshell while there in person? I meet with my PhD advisor quite irregularly so we arrange meetings on an ad-hoc basis depending on work progress, availability etc. Usually we communicate a bunch by email about the state of work.

I'm want to leave my course. I don't see how any minor changes to my work would change this decision. At this point I mostly just want to have a couple weeks of mental health break with no pressure to put time into the PhD, so I can have some brain-space to clear the remaining doubts from my head.

Should I let my advisor know that the meeting we are scheduling for later this week is about my position in the programme instead of normal project progress report?

PS. I am aware that a natural way to frame this would be to mention in an email that there I have some doubts about my work that I'd like to discuss. However that would be misleading - I'm pretty set on the idea that doing this degree is not right for me and the only doubts I have are whether this is the right decision at this time. I'm not interested in being convinced I should stay and try to "work things out" because my decision to leave is related less to problems I have with the course and more to simply having no reason why I should want to be doing it in the first place.
If you think that there is any possibility that the advisor can adjust your working arrangement enough to let you continue, then it would be better to give him/her a heads up before the meeting. That way thought can be given before the meeting and so it needn't be so ad-hoc. 

But have an idea of what you would need for yourself going in to such a meeting so you have some basis for discussing options. 

Leaving a degree is a big step, make sure it is the right one. And make sure that others have a chance to try to work it out with you if there is any way possible. But for that, they, themselves, need a time for reflection. 

And, as per your final thoughts in the question, have an idea about what you want the alternative to be. Ending with nothing isn't a very happy outcome. 

But the goal isn't to get you to change your mind. Rather it is to find a way so that the conditions are appropriate and the future is clearer.