We all should be doing some assistance exercises when it comes to training.  With CrossFit, those assistance exercises are sometimes ignored because of the compound functional movements that are predominantly used in most of your training session.  Below is a list of some exercises that you can perform 100 reps of into your training 1x per week.  These exercises help with injury prevention, muscle imbalances & weaknesses and also add some muscle tone to your frame.  In addition, they will help strengthen your compound functional movements.

In no particular order of importance:

Glute Bridges with feet on foam roller
Band Pull-Aparts
Empty Bar Barbell Curls
Kneeling Tricep Pushdowns with Band
Kneeling Straight Arm Lat Pulldowns
Empty Barbell Romanian Deadlifts
Walking Lunge Steps (50 each leg)
Hip Extensions on the GHD machine
Lateral Step-Ups (50 each leg)
Incline Plank Lifts off 12″ box

Pick 1-2 of these to do each day of the week.  You can superset them as well.  For instance, you can perform 5 x 20 reps of barbell curls and band tricep pushdowns back to back for a nice arm pump or 5 x 20 reps back to back of walking lunges and glute bridges to give your legs a nice burn.