Fitness

I used to do one from Freelitics that was really for a biginner/mid level.

For Time/5 rounds

20 Air Squats 10 Push-ups 20 Sit-ups 20 Superman/arches 40 Jumping Jacks

It was great because I could do it in 30 minutes, almost no warm-up, it activates the whole core, and can be easily modified to different levels. You can very quickly change the difficulty level of each exercise individually or increase reps. You can also try to improve your time. Finally, you can add weight with cheap dumbbells to every exercise.

I would usually change the supermans/arches for hip raises to engage glutes an hamstrings.

I would do it twice or thrice a week as a complement to some Natural Movements warm out, to get a full 1hr exercise. It was really easy to track progress too.