There are very few exercises that challenge your core muscles and improve your stability as well as the L-Sit. This is how you do it
Core Exercises: 6 workouts to tighten your abs, strengthen your back, and improve balance - Harvard Health
L Sit: How To Progress, Benefits, and Workouts
How To L-SIT (3 STEPS) For Beginners - Core Strength & Progressions
Give Planks a Rest. Try the L-Sit for Core Strength and Stability
How to L-Sit: Instructions, Tips and Progressions
Parallette L-Sit Abs Progression Gives Your Core A, 57% OFF
Core Strength Benefits and Exercises to Improve Your Workout - The New York Times
Parallette L-Sit Abs Progression Gives Your Core A, 57% OFF
From ZERO to L-SIT in 8 Steps L-Sit Exercises and Progression At Home