Invitations for children and an invitation for the Bishop, breakfast supplies (we have done continental or waffle bar)
We gather the children the first Saturday of the year and have them meet for breakfast at the ward house. We have all the teachers come and meet their new classes (sometiems in PJ's), so they each sit with their new class (we have done picnic style and tables). Then we have the bishop tell a story, sing a song or some other activity of his choosing. The kids love getting to know thier new class and also it is a chance to see the bishop and his wife (counselors could come too). We have each class paricipate in a small activity while gathered together. Some ideas we have done: class rules for the year, a class name, get to know you games (like the M&M one), taking pictures of all the kids to use for their birthday/spotlight, etc.
The kids always love the meet and greet and it is an easy activity as you want it to be.