The “Create Manual Activity” window from the Garmin Connect Calendar has a link called “Add more data”. Clicking that link results in a screen which has only two more fields which display: Time Zone (not typically changed or needed) and Avg Speed (a calculated field). Then you have to click “Add more data” again to actually add more data to the activity.
I suggest changing the “Add more data” link to point to the Manual Activity page that has all the other data fields (Elevation Gain/Loss, Max Speed, Avg/Max HR, Calories, etc.)