What do you think of when you think of core train? Some may think of sit up crunches or rush and twists early on when working with clients. Sometimes I'll start with anti movements or anti rotation, anti side bending, anti flex. This is a great way to just build up capacity, some more dynamic movements as we progress through the training program.
Here's some examples of each. The poff Press forces you to resist rotation. The side plank forces you to resist side bending. The plank forces you to resist extension.