To rename an Activity or Folder, click the checkbox to the left of the item, then click the 3-dot menu on the Activity Toolbar that appears. From the 3-dot menu, click "Edit Properties." A popup will appear allowing you to type a new name. Click "Confirm" to save the new name.

BONUS: You can also edit the Description of an Activity.

BONUS: You can follow the same procedure to change the color of a folder's icon.