One simple thing I found that was rewarding (cost nothing) and meant a lot to our clients was to give them the day off of their birthday. With all assignments completed and there has been no problems in groups session, they are excused from group and any group assignments given on that day. Its allows them to know that they are still worthy of being "celebrated" and their peers take notice and offer their best wishes. As staff, I think we would enjoy having our birthday off work (excused)...wow.
Small, free, but means a lot in the long run!
Also, to promote teamwork and re-enforce lessons we sometimes play Anger Bingo, Parenting Bingo, and the team that wins get to have their choice of what day they would like to be excused from group (mostly Mondays) and they become excited about the chance.
Brenda
Mississippi