I did a 24" x 6" marble shelf along the back wall of the tub that fit into the corner on one side and was rounded on the other.

You could just do:
http://products.daltile.com/series.cfm?series=53

Or something from Plum Street

These are super basic but it was the classic choice for decades.

Here is a link that might be useful: Plum Street

Yes, those built-ins palimpset posted would give a very authentic vintage look. In my new subway shower, I have a niche but also added a small corner basket-type shelf in polished chrome - Hotelier by Ginger. Got it on sale, and it was a perfect supplement to the niche.

I used Ginger corner baskets and love them. I used 2, one over the other. Same size, the bottom one holds the soap. Lots of big bottles fit in them.

If we have well water the Ginger ones may corrode, right?

"If we have well water the Ginger ones may corrode, right?"

No, they won't "corrode" but they may get rust stains or other mineral deposits on it. Just like all of the innards of your pipes and other plumbing fixtures.

If you have well water, you need to get it tested and treated accordingly. Or else you will have an ongoing problem with maintaining your home's water contacting surfaces. And all of those expensive faucets and shower valves may need to be bought again.