Secret pages rock! I think they're one of the best things about having a personal site (besides everything else that is awesome). I like stashing treats for people willing to search for them, as well as pages I want semi-private (meaning, I've mainly made them for my own use, but I'm also fine with someone else seeing them if they're curious enough to search).
I tend to hide my secret pages in plain sight within images, but also if people don't want to go on that hunt they can also just check out my sitemap, where I've pretty much listed everything on my site besides individual blog posts and sub-gallery pages (that would get too long). Arguably my secrets are not that hard to find, but I still consider them pretty secret since I do not spell out immediately how to find them on the page where I have hidden them. I have some more secrets planned for some of my galleries which I don't plan to even have listed in my sitemap, so there will be more difficult hunts in the future.
Also the way I have my secrets stashed is meant to work with the light narrative of my site, which is that it is situated on (or technically is) a large branching mycorrhizal network that resembles a tree floating in space. Each of my root pages have two white branch images placed on the z-index that... well :) that would give away the secret.