I have planted Carex ‘Evergold’ in one of the small, square beds set into the gravel and I think flowers would be interesting standing up above the grass. This is the ‘hot colours’ part of the garden and so I think quite a dramatic combination could work well: Astrantia, Sanguisorba, or a purple Sedum.
I have planted low-growing grasses in small, square beds and think they will need some taller, wavy flowers planted amongst them. But which flowers? I have combined Luzula Nivea and Carex ‘Frosted Curls’ and I think blue flowers would look good standing above the grasses, and perhaps the cornflowers are best as this is a cottage garden.