I think this has to be my favorite line of the Girl Scout Law! 

The thing about this petal is that there are thousands of ways one can do this! 

I would definitely leave it up to the girls on this one. Ask them what it means to make the world a better place and have them come up with ideas they can do as a troop. If they are having trouble coming up with their own make some suggestions or give them starter ideas. When girls talk about and feel a connection with the cause it's much more meaningful to them and they will gain more out of the experience than just picking something for them.