Here's a way to make virtually any core stability exercise more challenging, without adding additional load or requiring any additional equipment:
Add an exhale component to the exercise.
So, instead of doing a plank for 30-seconds, you might do it for the duration of ten exhales. These exhales should be intentional and forceful, breathing out fully and completely, and contracting the abs hard + pulling the ribs down on each exhale. Yes, people around you should be able to hear it.
Give it a shot, it actually makes it a lot more difficult than you'd think.
Here are a few examples: