Remember the Spirograph toy? I bet you can make various designs with that and just color them in
A very long time ago, I discovered that I could make my own adult coloring patterns with a compass and straightedge. These are strictly geometric patterns, not the really elaborate cats and nature pictures and such that I've seen in stores, but they have two advantages.
I made my own coloring book way back in second grade with a ruler and pencil. I just made pages of various shapes and colored them in later. I distinctly remember a classmate taking one of my handmade coloring pages and showing it to the teacher (why I don't know). I will never forget the very disapproving look the teacher gave me
Yes, you do need an account to actually use Pintrest. You can search for for stuff and see some of them but there's this big annoying sign in / sign up box that covers most of the page that won't let you do anything more with your search result. I think Pintrest works more like an album. You save (pin) stuff that you like to an alubum and then you go through the album later to get ideas for projects and whatnot.
Also, someone here mentioned that they had found a bunch of coloring pages on Pinterest. How does Pinterest work? I've never signed up for it. Is it like Facebook where you basically have to sign over all privacy rights?