When I managed a large team, I had custom 'Thank You' cards printed that were blank (you could also use store-bought blank Thank You cards) and handwrote a specific message for each individual noting down what I appreciated an signed them. I would leave these notes at their desk either before or after their work. It was always a surprise for them and many of them treasured these notes and had them pinned on their walls for several years.

A sincere 'Thanks' in the presence of their peers also works.

You can do this with all work colleagues who have played a significant part in an initiative that you are leading/responsible for, including those outside of your department.