Pyramid Equestrian said:
What I do is I two-point a few laps each direction after I ride while my horse trots out

Cappuccino said:
planks, squats, deadlifts, dead bugs, bird dogs, weighted windmills, hollow holds/rocks, Leg raises, L-sits etc

You're going to want to do things that encourage bracing and work on anti-rotation. You can do sit ups till the cows come home but your core is so much more than just abs. It's the whole 360 mid section. Flexion exercises will only go so far, got to add all facets.