I used to try all the abs exercises for fun. I started doing those ab exercises for gymnastics but if you are bored with traditional sit ups, try some of them in your core training routine.
Leg Raises (Seated)
This exercise requires a certain level of flexibility. Starting from a seated position with straight legs and raising the legs. Originally, I was doing this to master L-sit, but put this core training in my training routine. Great exercise for abs as well as the hip flexors.
Hanging leg raises (pike)
You may have tried bending knees, If you are a training geek. Try this, if you like the challenge. This is not swinging but instead you want to use rectus abdominis to tuck your ab region (six pack muscle). Also make sure the lower back is not hyper extended while bringing the legs down. Engage the core muscles throughout the movement.
Rocking chair (Hollow)
Lying down with your face up, lift your arms and legs slightly from the ground and you are moving the body like a rocking chair. Like doing dead bug, you always want to be careful that lumbar lordosis during this exercise. Excessive lumbar lordosis may cause lower back injury.