Hi Kim,
I can only do so much without actually designing it, but from the pictures, I would recommend the following.
I would definitely do bluestone or brick, dry laid to keep the cost down. If you do more concrete, even though you had a good thought to put grass in between, it will never match and look like an after thought. The bluestone or brick will be pretty and add interest. I think it will look fine because of the shape.
I like your idea of dividing the area with grass. However, a narrow strip of grass will be difficult to care for and will most likely easily
burn out between the concrete and the stone. Instead, why don't you consider a strip of groundcover, about 6"wide...easy to care for and
pretty.
