A Crossfit'ish workout from strongmanwod.com. Try it. (hint, it's not the weight that makes it hard).
Squat cardio:
225 squat x 5 reps Every Minute On the Minute(EMOM) x 5 minutes
Rest 1 minute
275 squat x 4 reps EMOM x 5 minutes
Rest 1 minute
315 squat x 3 reps EMOM x 5 minutes
Rest 1 minute
365 squat x 2 reps EMOM x 5 minutes
Rest 1 minute
405 squat x 1 reps EMOM x 5 minutes
Squat cardio:
225 squat x 5 reps Every Minute On the Minute(EMOM) x 5 minutes
Rest 1 minute
275 squat x 4 reps EMOM x 5 minutes
Rest 1 minute
315 squat x 3 reps EMOM x 5 minutes
Rest 1 minute
365 squat x 2 reps EMOM x 5 minutes
Rest 1 minute
405 squat x 1 reps EMOM x 5 minutes