You can now create manual activities for unrecorded stuff. These are also downloaded from Garmin Connect and Strava if you create them there.
Add a calendar entry by clicking on a blank area of the calendar or the plus button on the toolbar. Choose an activity type:
Then change “Workout” to “Manual activity” in the category drop down and complete the other fields:
The new activity will appear on the calendar (in grey because it doesn’t have heart rate or power data):