The celebration is a time to give recognition to the team. Special hats or shirts are sometimes given out, and the team members can wear them with pride.
Certificates of completion are customary, as are team pictures. Light-hearted awards may be given.
Invite people who may not have formally been on the team, but who helped make it a success. Maintenance, purchasing, logistics, food and beverage people are all likely participants.
Try to get some PR out of the event as well. If there is a company newsletter, get an article on the Kaizen written up. Make sure that Union representatives are invited, if this is a union shop.