How do you tell a leader they've been a good leader without sounding sappy?

If you truly want to thank them, jist say thanks for your leadership and guidance. It's important to our team and we appreciate what you do for us. Most likely he will say thanks and he appreciates your support. If he is a great leader he would tell you that he can only do his job because of troops like you! No leader is a good leader without the support of his or her troops! The troops get the credit!

Simply mention the accomplishment. "since you took over, I've noticed X. Thanks" That's all you need.

I can tell you as a leader that the items I treasure the most are the tokens that they gave me for a job well done. A plaque, an engraved K-bar, something unique to the situation, a framed guidon.. all are treasured items that I keep on my wall at home.
Once in a while, one of my former Soldiers pops up out of the blue and contacts me seeking advice, usually accompanied by how much they enjoyed serving with me and my unit. I am not vain, but hearing that can't help but make you feel like you are doing something right.