Those exercises circuit is the one I use for my training as well as for personal training sessions. Try it and if it is too easy, add more duration/repetition and sets to feel the burn!!
Plank saw
start 
end
The reason why I love plank is that if you perform correctly, it is possible to keep your spine neutral all the time. This is so important while training. This plank is an advanced version. An alternative way is using a foam roller on your shin instead.
- Duration 25 – 30 sec
- Set 3
Crunch (feet on a chair)
I wouldn’t recommend doing sit ups but crunch is fine. Directly target rectus abdominis (six pack muscle) . Make sure you don’t pull your neck (it may cause neck strain), but just support it.
- Repetition 12 – 15
- Set 3
Deadbug
Another great exercise for your abs. Yet this is also difficult to perform safely.
Again the idea of training safely is keeping your spine neutral.
As a leg goes down, the leg will pull a hip forward that makes a position of spine susceptible to lower back injury. This is the reason I don’t recommend bringing both legs at the same time but just one after one.
- Duration 30 – 45 sec
- Set 3
Crunch
Another crunch to push you harder for the day. I set repetition 12 – 15. But if you want to feel the burn, push it till your limit to end the day!!
- Repetition 12 – 15
- Set 3
Click here if you want to see it as a video.
In-home training Back muscle here.