Since this is a brand new watch, and shares a lot with the 5 and 5S, I've been keeping an eye on that forum too. I saw a neat trick there, and had a chance to try it out last night. You might enjoy this, too.
Set up:
(1) Hold the UP/MENU button to go into the menu
(2) Go to Settings -> System -> Hot Keys -> Hold START
(3) Set it to "Change Sport"
That was easy! You could assign this to a different key but start is fitting.
In use:
(1) Go run a mile
(2) Long-press start
(3) Choose walk
(4) Start walking
Results:
You get one overall activity with combined summary stats (time, distance, vert, calories) and detailed sub-activities.
https://connect.garmin.com/modern/activity/1675911867
Your profile and reports on Connect treat each leg or phase or sport as a separate activity. I did two run intervals and two walk intervals, this counts two runs and two walks.
Strava apparently takes each leg as a completely separate activity.
Use cases:
Run/walk intervals. Avoids counting walking time as running.
Rock climbing. You often hike to a cliff and then set up and climb. I had a partner who insisted on always having a GPS track of the approach hike; he said if something goes wrong, we don't want to rely on memory to get us to the car while we're in a panic.
Biking to the beach to swim because parking is a nightmare.
Mutli-day backpacking, maybe? Treat each day as a separate hike but keep the entire trip under a single umbrella. Not sure if this works if you change to the same activity.