How do I create an empty activity? Creating new ones is easy enough, but they have the same windows as the ones I currently have open.

Alternatively, how do I move all open windows (either across all activities or in the current activity) to be only in the current activity? This would have the same effect, so either solution is OK.

Thanks!

I suspect something might be up with your setup somewhere as the default is for application windows to open on the current activity.

You are in fact running KWin, right? (ctrl+Esc to bring up the process manager, search for KWin). If not, you should switch back using System Settings -> Default Applications -> Window Manager. Then either log out and back in, or replace the window manager by running: "KWin --replace", after which you'll need to reassign the windows to the activities you want them in.

(assuming you are running Kwin) You might check for Window Rules in System Settings -> Window Behavior to see if there are any rules forcing windows to be on all activities.

Also, you might check using a new user to see if the problem exists just in your user's current setup vs. a system-wide problem.

For assigning individual windows, right-click on the window title (or hit Alt+F3) and use the Activities submenu.
For bulk-moving the windows to an activity, you should be able to do that using a window rule.