One of the things I do is have the students stand in a circle. They have to keep stepping in and in until they are shoulder to shoulder. Finally they have to all turn to the right and keep stepping until they are chest to back. Then on the count of 3, they have to sit in the lap behind them. It is hysterical and it relaxes the kids and helps them to get to know each other or the new kids in class. It is not as easy as it sounds. It takes a lot of concentration. The circle has to be round and tight.
No rules! I always found that if I started to "teach" as soon as the kids sat down they were caught off guard and became interested in the subject, rather than concerned about the rules for the class. I'd get to the rules and seating and stuff later on...but always start with something interesting. What might you use that will be interesting...tell 'em you will be having a class Web site and need ideas on what should be on it.