Set new/additional waypoint or course point from the point of interest list (eg restaurant) during activity?

you could select it and will nav to that point, then could select the waypoint '/ route of your previous course and get back onto it.

I know that when creating a route on the watch you can add waypoints, however don't think you can edit once its running, though above suggestion works and have used that in the past. Just make sure your routing is set to map and not to course so it can recacluate the directions to get you quickly back to the old waypoint, rather than having to get back onto the course.

its been a while since I watched Hiking Guy's video on garmin navigation - but perhaps he covers it.

thank you. Yeah, interrupt the old navigation, select the new point, and then, after reaching the new point, (re)start the prior navigation is exactly what I did.

 I'll check out the hiking guy, thanks for the suggestion.