Select a long song 7-10 minutes or 2 songs back to back. I ask the class to select a resistance that will challenge them for this amount of time. We use a scale of 1-10 and I tell them "5" is a good place to start. It should feel like a moderate seated hill with moderate cadence.
Students will ride the entire time at this cadence/resistance. The challenge is to choose one that you can keep up for this long.
I like to use visualization during these segments. Sometimes I choose a place locally that we've probably all ridden on and then I might select one student at a time to stand, increase resistance and sprint for 30 seconds to "lead" our group. Letting each person have a turn.
Other times I just let the music take us away and remind them to focus on cadence.
This is a very effective endurance segment and a lot of my students like this because it's a break from the anaerobic workout that is so easy to fall into during these classes.
Remember that people don't have to be jumping and hovering and sprinting all the time. These kind of segments will make them stronger.
Keep spinning.