You have so many different options. If you want to change your interval times, you can do just about anything from Tabatas (20 seconds work, 10 rest—8 rounds for 4 minutes) to 30, 45, or 60 seconds of work and your choice of rest period. You can insert a “theme” into any one of those. Leg day, upper body day, etc…
Try different pieces of equipment–or maybe just one piece of equipment for the day.
Also, check here on IDEA for some great ideas for your classes.
Hope this helps.