You can quickly turn an activity into a sub-activity of another activity.

Method 1 (drag-n-drop)

1. Open Activity Center.
2. Select the hierarchical view mode.
3. Locate the activity you want to turn into a sub-activity of another activity.
4. Drag the activity onto the target activity until the latter gets surrounded with a thick black rectangular frame.
5. Let go of the activity being moved.

Method 2 (via the details section) 

1. Open Activity Center.
2. Select the activity you want to turn into a sub-activity of another activity.
3. In Activity details section, hover the pointer over the name of the activity until '– change' appears.
4. Click '– change'.
5. Click the activity to which you want to reassign the current activity as a child.