Planking, ab crunches, and swimming are amazing to develop a strong core. Swimming also develops general strength and fitness 😊
bike riding and planking is really good
Plank is soooooooooooo good it helps with a straight back too!👍
Plank, mountain climbers, incline sit ups ... Run hills
Download the app "Sworkit" and do 10/15 minutes of straight ab work everyday. (You can pick which ones you want to do and then it times you for each one). I've found leg raises, 6 inch holds, abdominal crunches, full body sit-ups with a 10/15 lb medicine ball, and incline sit-ups with a plate really make a difference in core strength when riding or jumping bigger.
Pilates is amazing for core and overall body strength and hip suppleness. An exercise ball and exercises specifically for equestrians (available online) is also effective.
I always do a few laps of 2pt at walk trot and canter during my warm up. Try some transitions in 2pt too, it's harder than you'd think and really helps in learning how to maintain balance
Planks sit ups and you can also practice holding your 2-point seat at home and see how long you can hold it for😊
Planks and sit-ups also if you look up equestrian workouts online there will be lots of good workouts for you.
No stirrup work and riding in two point will definitely help you build muscle. Outside of riding, planks and basically any variation of sit-ups/crunches (there are a LOT out there) will help build core strength.