Between bike rides I frequently hike around the trails near my house. I see that the new interface supports manually adding activities, but how do I get calories (or heart rate, other metrics for that matter) entered?
When you create a manual activity in the classic Garmin Connect, it allows you to enter Time, Distance, Elevation Gain/Loss, Max Speed, Average/Max HR, Calories, Average/Max/Min Temperature.
I don't know what the interface looks like in the modern Garmin Connect, but you should be able to manually enter activities in the classic Garmin Connect here.
If you want more data than that, you'd need to manually create an fit/tcx/gpx file that contains the information, or you'd need to use a Garmin Fitness device. Why not just use your existing device when you do your hiking?
Yeah but how long are the Garmin software folks going to support that classic interface, one day it will be gone for sure. They need to support "add manual activity" to the modern interface.