What are some more exercises to activate the core?

Ana Lle F.
curl ups, knee planks (planks but with the knee on the ground)

planks, twist planks, sit ups, bicycle, etc

Amy P.
A plank is great as it works the arms, core and back. You could also try crunches, leg raises and scissor kicks. There at lots of great workouts on YouTube. Try emi wong.