Label "activities" in Plasma 5 desktop?
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.