This is something that SportTracks does really well, and shouldn't be hard for Garmin Connect to implement too.
When an activity is completed, an attempt should be made to match that activity to a workout scheduled for that day (match on activity type, and possibly other things like duration / distance). Once matched, the user should be able to review the match and correct it, in case there was a mismatch. The notion of marking a scheduled workout as "completed" would have to be introduced, along with the association between that workout and the activity that "completed" it. Again, users should be allowed to review the matches, and even manually make matches on their own.
This should be especially easy if the user, on his Garmin device, is using the "Do Workout" option. In these cases, there would be no ambiguity about the "match". But still, reviewing / changing matches should be allowed.
You could then show the user things like:
- You planned to run 50 miles this week, but you only ran 40.
- You had hoped to complete today's run with a 8:30 pace, but you only did an 8:45 pace.
- Etc.