I also have moved away from ‘traditional’ ab exercises; however, I like to employ techniques in which people need to use the abs in the manner for which they were designed, i. e. as stabilizers.
A favorite of mine is the 1 Arm Chest Fly, which can be done lying on the floor, on a bench, on a Bosu, on a ball, on a foam roller …. you get the idea. I always cue people to be in a neutral pelvis, engage the core and have the objective not to move the body. As soon as the unilateral load is applied and moves outside the body, the client needs to deeply engage all core muscles in an effort not to move over to that side. What I like about this exercise is that people give me the feedback that they can ‘really feel it’ and thereby developing a better connection to core muscles.