For this age group it must be fun! They are at an awkward age and socializing is huge.
Playing music would be great!
Short drills incorporating jumping jacks, jump rope, bunny hops, frog leaps, bear walks, crab walks around the bases are fun along with agility ladders, medicine balls, lunges, squats and pushups.
You could work base to base with teams, each doing a different drill or do relays, they are always a hit!
I would incorporate group games, partner workouts, “ice breakers” where they have to say one thing good about the person to their right in terms of how they play softball.
I suggest pairing the “uncool” girl up with the “cool” girl and show them that people are people!