A simple power-lifting exercise, the Bench Press, is excellent for improving chest muscle. However, CrossFit appears to prefer overhead rather than bench pressing, most likely due to the transition of overhead work to Olympic Lifting, but perhaps also due to the risk for having weak balance and rigid shoulders to perform too much bench work. Given the high degree of versatility needed in CrossFit skills such as Olympic Lifting and gymnastics, this may be counter-productive. But just make sure you combine some Bench work with a lot of back chain work.
Five sets of the triple,
Sit-up
Back extension
Bench-press 5-10RM
Move methodically, deliberately, with precise SLOW movement.
Don't hurry; minimize the cardiovascular quality.
This is strength training only! Make each effort a serious challenge.
Notes:
1. Bench is at easy pace to maximize lifts.
2. Match each rope assent with ten push-ups.
3. Rest before each of the three distinct efforts as needed.