 Balancing and keeping up people's energy and enthusiasm throughout the whole session is a very difficult and very important thing to do. If your workshop is only two hours long, you could start with a light introduction or energizer activity at the very beginning, but you might not need another one. If your workshop runs over multiple sessions or has a long break in between, for example a lunch break, it's really important to bring people's energy back up when they come back after a long pause. So use quick energizer activities after there's been a long break or if you have multiple sessions running over multiple days.