Having a strong core is essential if you do CrossFit because it makes you feel like you can lift more, run faster, and jump higher. Although we rarely do straight-up ab exercises since most CrossFit workouts are full body, when we do work our abs, these are the core-strengthening moves our coach has us do. Even if you don't do CrossFit, you can still do these exercises to get a lean and strong core.
In a CrossFit class, substitute two gymnastics rings for the TRX.
Use sliders or two hand towels on smooth surfaces, or paper plates on carpet. If you don't have enough space to walk your plank, just do plank ups with sliders. If you don't have sliders, towels, or paper plates, just do mountain climbers.