bankrupt, I am in the same situation. There was a garden featured in Chelsea Flower show this year and they had this gorgeous little plant (growing to I think a max of 2" height) between paving slabs. It is called Kidney Weed (Dichondra Repens). Looked it up and there are a couple of varieities, the lime green one with kidney shaped leaves is the one I am after. I have tried to find seed in the seed shops but no luck so far. You could also try Golden thyme or Camomile, which is quite sturdy underfoot. Let me know if you find something good.