An appropriate message to write in a leaving card should express how much the writer enjoyed working with the person leaving and grant fond wishes for a great future journey. The message can be a long, but it can also be short such as, "It's been great. Best wishes for the road ahead." It is also appropriate to include contact details if the writer wants to stay in touch with the person leaving.
When writing a message in a leaving car, the writer can reference projects they worked with the other person on or fond memories that they have of them. They could write, "I'll always remember the Christmas party of 2007 – you made it memorable!" or they could write, "your hand on the Smith-Martin deal was invaluable, and I look forward to seeing what you do once you leave."
All of the messages in a leaving card should be pleasant, and if the writer does not have anything pleasant to say to the person leaving, it is acceptable to simply write "best of luck" and sign their name.
In lieu of writing their own text, the person filling out the card could include a leaving poem or sentiment. The Gaelic blessing, "May the road always rise up to meet you" is an appropriate sentiment.