Boxing fitness classes can keep things interesting while getting a great workout mixed with self-defence. Add some Muay Thai, K1, Capoeira or Taekwondo for some deadly combos. Skipping is great cardio, 2-3 minute rounds (30sec -1 minute sprint at end if keen) 30sec - 1 minute rest repeat.
Combat fitness classes will teach you to fight and get a great cardio workout at the same time. Learn how to do flying kicks and knees, back fists, spinning elbows and more.
Also a good instructor on the pads and shields can keep things dynamic and you focused and pumped up. Awesome cardio.
Squats and lunges can get boring, try alternating push kick squats (helps with a kick shield and partner) and step back lunge, forward knee (same leg) in alternating reps.
Too many times at the gym I used to think, "what am I doing here?" But training to potentially fight to defend your life? There is no stronger motivation.