Here's one I haven't seen bother the neck, the Pallof Press. (Or, because I hate exercises named after names, what I call "Ab Punchouts"):
It’s crucial to make sure the hips / spine are not moving. If they are, we’re defeating the purpose of the exercise, which is to prevent torso rotation. For instance:
The abs are often worked as a generator of movement. However, for reasons beyond the scope of this post, it's better to train them as a preventer of movement. What we're doing with Ab Punchouts is using the abdominals to resist the torso from being rotated to the cable. (Hands should punch directly in front of the chest. Not get pulled inward.)
Personally, I find the standing version of this exercise hard to get people to do properly.
First off, getting people to accept the notion of doing an ab exercise where their abs don’t move is work in itself. Getting them to know when they’re moving is more work, getting them to then understand how to prevent the motion yet even more work.
It’s not that it can’t be done, I just think there’s an easier way. I put people on the ground:
This works wonders. Now all you have to say or think about is, “Don’t let your body come off the ground, at all.” (One often feels one hip being pulled off the ground, to the cable.)
Furthermore, people realize how much their lower back moves because of how much concentration it takes them to not move. Once they go to a standing version of this they have more understanding of the goal of the exercise.
Lastly, it’s typical for one side to feel harder than the other. This is especially true of those with any type of rotational sport history (tennis, baseball, etc.) or those with lower back issues (imbalance between obliques). Hence, a major purpose of the exercise- to even things out.
Weight wise, with a cable stack starting at 10lbs, I don't have many everyday people who go heavier than 20-25 lbs. I have them do 3-4 sets, 15 reps per side. If no cable stack, you could also hook up a resistance band.
Credit: Another (better?) Pallof Press variation
Tangent: plank variations can take one quite far. Whether that's abdominal strength, or even fending off boredom. The variations are practically endless. Here's one I used with a Canadian professional football player (strong dude), but this was still challenging: