Inshort: practise various forms of the shot in a fun competition format.
Organisation: each group a basket and a ball, the baskets are preferably (but not absolutely necessary) arranged in a circle or rectangle. The number of people per group is less important (all groups should be about the same size).
The first assignment for the groups is: make 10 walkthrough ball goals. When you are done, the creator of the last goal goes to the trainer to get the next assignment. Which group will have completed all assignments first?
The trainer walks around the room, encouraging or correcting the players. He has a piece of paper with a list of assignments. When someone comes to get the next assignment, first ask him which assignment he just completed (after a while this can vary considerably) and then give the next one. Here's an example: 10 walk-through balls, 15 penalty shots, 5 shots from 8 meters, 10 walk-through balls from behind the basket, 5 dodgeballs next to the post, 10 shots from 6 meters. Everything is possible of course, a lot of speed will come when the number of goals to be scored is kept small.