How about tin "ceiling"tiles?

The darker walnut tumbled travertine would not be too matchy at all, especially if you enhance it. I would not do the lighter travertine. See attached picture of a darker stone/lighter cabinet.
I would try to find a special glass somewhere that would pull out the colors of the Cambridge, it has so many good colors in it you could accent with. Try to keep them subtle and minimal, but use as an accent.

How about a tumbled travertine mosiac? Also, I think the island color isn't working with the floor and cabinet (too red). How about a darker stain for the island? Check out this mosiac travertine and the lower cabinets a darker stain. I think the darker stain for the island would work beautifully with your new counter, floor and cabinetry.