Bag of 25 lapel pins representing the DA40. Give one to all your instructors, and also to LAPL/PPL clients upon them soloing on a Diamond. You can further give the DA42 pin to clients who enter the Multi-Engine stage of their training, to keep up their appetite!

NB about the etiquette: this should not be worn on the red Aeropole jacket, a sweater or wind breaker, but only on clothing with a lapel to neatly stick it into: a suit jacket or blazer.