Burpees! Lots and lots of burpees! Working out the core on Day 4 with 5 rounds of burpees. If you want you can do 10 or even 15 rounds.
Good luck with your workout!
Disclaimer: Do not perform these exercises if you have any injury, get rehabilitation first. If you get short of breath, feel fatigued or tired during any of these workouts, stop and take a break. Consult a doctor/trainer if necessary before starting any workout session. Children should do them under supervision of an adult.
These workouts are taken from the Darebee 30 days of HIIT program.