Have everyone write down one thing they are truly grateful for on a small piece of paper during a team meeting (or just around the collective office one day).
Then have everyone pass their gratitude from person to person several times around the room, stopping at some
random point when everyone then reads the one they end up with aloud to the group. The added benefit of this exercise is that you not only gain the proven benefits associated with writing down a gratitude, you also get the multiple whammy effect of reading what other people are grateful for.