The title bar of an application window currently has only "Show In Activities" for sending an open application to an Activity window.  I recommend that, as with "Desktops" it should have the same options - specifically with "Move to Activity" which would be far more useful than "Show in Activities" which leaves the application open in both the original Activity and the destination Activity window.
Also, it would be wonderful if the each Activity window could remember the application windows that were open from the previous session.
That said, I give credit and congratulations to the amazing work of the programmers who have built KDE Plasma over the year to a state-of-the-art desktop.