If in full sun there are various Thymes (Thymus) that are simply perfect for the job. There are quite a number of other plants that work well too of course that are not necessarily related (thank goodness for spell check, still can't spell that last bleedin' word). Can you say if in full sun or in shade?

If in full sun....the stonecrop sedum makes a bright limey green mat with star shaped lemon coloured flowers in June.

Erigeron karvinskianus, small violas, small sedums like S.acre, small saxifrages. You could look for alpines in you local garden centre and get a few cheaper ones that you like. It depends how much they're likely to get walked on, but I wouldn't experiment with expensive plants that may be too fragile and die off, or make a take-over bid and have to be given the heave-ho

Creeping thymes are very good for accidentally-on-purpose treading on, you get a lovely waft of scent!

I have chamomile between mine

sempervivum tectorum or common houseleeks come in a multitude of colours and look fabulous in the cracks between paving slabs - they also flower. Or how about a small variety of campanula - the name of which escapes me.