If we are talking about hidden items then I would have to say the J.C. Boathouse, and all the "little donations" that Skippers have left over the years. For an older Skip it is a trip down memory lane.
"Happiness is a Low Water Level"
"Creating magical memories and making Managers cry since 1955!"
I found an unofficial HM in the Jewel of Orleans shop. How do I know it's unofficial? The CM told me so, after I had gotten quite excited over seeing one I did not know about (I know, I know. Never take what a CM says as absolute fact. This, though, seems minor enough, that I think it's true). I still think it's neat, though. In fact, here's the photo I took: