How can I decorate a bay window?

My parents have same type of window with custom made short shears on the inside

Personally, I like curtains to the floor and I would add them to the outside of the window box.

LOL! And I would vote for curtains inside the bay so that you retain the look of the shelf, etc. Hanging curtains inside makes them look very custom as opposed to a rod across the entire top imho.

I saw a post here where they used stained fence pickets across the top of the frames inside the window box. Then, hung industrial plumbing pipe and flanges along the tops of each window to hold the curtains.