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.
When I do the full P90X Classic program cycle, I really do enjoy the Plyometrics, Kenpo X, and Core Synergistics routines from P90X. However, they are very long routines (an hour each) and demand way more than is necessary on a normal basis outside of the 90 day program.
On the months in between when I do the program, I usually pick and pull my favorite exercises from these and assemble a mix to do a 25-45 minute routine on the two cardio-based days between my strength training days, yoga/stretch day, and rest day.
The killer Plyometrics routine is still up for free on archive.org. They also show how to modify each exercise to deal with any specifics you need, such as modifying the exercise to protect your joints if you have bad knees, dialing it back if you have a bad leg, etc., which makes it worth skimming through even if you're already familiar with the exercises but want to find ways to make them safe, tougher, or just different.
The Core Synergistics routine is also still up for free on archive.org. I don't do these exercises nearly as often as I should. This was only scheduled a few times within the Classic P90X schedule so I'm just used to not doing it often (one of the flaws they fixed for P90X2 and P90X3 was scheduling in more core specific stuff throughout the weeks instead of testing it with this Core Synergistics routine a few times in 90 days). I usually like saving cardio type days solely for cardio and just engage my core during regular exercises and yoga poses to help build it otherwise, but if you really want to add some core focused stuff into your HIIT or circuit, this has some good ones. I particularly like "The Dreya Roll" as a fun change up.
If you're looking for things to incorporate or a grueling routine as a whole, scroll through the videos to see if there are any exercises that look fun, challenging, or really hits the areas you want to work, and mix them into your own sessions.
Keep in mind, whether picking and pulling or doing the full routines themselves, that many of the warm up sections at the start of P90X classic videos are outdated. If they include static stretching, which has more recently been replaced with the recommendation to only do ballistic stretching before the workouts, modify as needed and save the static stretching for after the workout. Also, if you find a copy of Kenpo X, it's a cakewalk unless you really pivot and twist with each punch/kick and engage your core as if protecting your stomach from any incoming hits from an imaginary opponent -- then it's a really fun and tough workout.
Edit: The reason the demonstrator named Erik looks so familiar in the Plyometrics video is because it's actor Erik Stolhanske, best known as Rabbit in Super Troopers (2001)/Super Troopers 2 (2018) and Todd in Beerfest (2006).
I row on HIIT and my rowing machine can be programmed just fine for that. It's whole body, barely any impact on joints and I can watch stuff during.
I usually program in 5 min warmup + 24 × 30s / 1min rest intervals. Feels just about right for me.
Here is my HIIT programme, I really like intense workouts. But you can use and send me your info what the workout is like. Hiit training my
