I've been running Plasma 5 as part of Kubuntu 20.04 since December 2020, and I've set up "activities" after reading suggestions that doing so helps with actual work efficiency (reduces distractions as well as limiting desktop clutter). I currently have four activities defined: Daily, Games, 3D CAD and Printing, and Photography -- this allows me to minimize the number of desktop icons in each, as well as have task-relevant wallpaper. As required to make desktop links specific to each activity, I've created folders within /home/Desktop for each activity.
However, I often run software (like my browser) maximized, meaning I can't tell at a glance which activity I'm in (mainly an issue if I'm called away from my computer and need to resume my mental processes) without minimizing one or more windows in order to see the wallpaper. I can tell, also, by clicking into the system launcher, but that depends on my memory of what favorites I have in which activity (hence, if/as activities proliferate or my memory deteriorates with age -- I first used a computer in 1975, in high school -- this will become less reliable).
Is there a way to put an "activity label" into the task manager bar so I only have to mouse over the proper screen edge to remind myself what activity I'm in? One option that suggests itself is to create a tiny program that has an appropriate title and is set to auto-launch in each activity space, but I'd prefer something that uses either existing widgets, or a label function built into the desktop environment.
Aha!
It occurred to me after posting my question that this can't be an exotic request -- my memory is better than most, so I can't be the only Plasma Desktop user who needs a little nudge keeping track of the activity I'm in.
It turns out that the pre-installed Plasma widgets include a number of simple items to keep track of current activity; I chose "Activity Bar" which gives a plain text list of the available activities, with an underscore for the current one. Not only does it allow a quick reminder (just by a mouse-over, to pop out the hidden Task Manager bar), it gives a one-click activity switcher. There are variations, Activities and Activity Pager, that give more graphics, but this Activity Bar lets me put the list in my secondary, top-of-screen Task Manager (where I keep my weather, system monitor, temperature displays, and so forth) without excessive clutter.
Problem solved!