Machine lifting? Free weights? Yoga? Medicine ball?
Long distance running? Sprints?
If you could only pick one...???
My daughters do sit-ups to strengthen their cores and work out with power bands. It's like weight lifting, but does more to get negative resistance and they can take them anywhere. Their coaches have them doing carbo in the gym, but it's not something they choose to do on their own.
If you can only pick 1, swimming.
Otherwise a balanced program, specific for position.