Zone 3 Paving Stone Filler Plants

Last fall we created a small patio-like space and deliberately left extra space between the paving stones so I could plant there. I already plan to plant creeping thyme and was wondering what other herbs I could plant that would be able to take some trampling. They need to be hardy because I am a generally lazy gardener and because we have tough winters here in Winnipeg, MB, Canada. The space gets lots of sun and good snow cover in winter.
I would prefer to order seeds rather than plants, if possible.

Creeping thyme might not be winter hardy in Winnipeg, since most authors rate it as hardy in zones 5 to 8. Wild thyme is rated zone 4 to 8 but some authors rate it zone 2 to 9 and might do better for you. It is also the only thyme we have as seed. Some other herbs which form mats would be Achilleas and Ajuga reptans . You might wish to consult the Q&A section of our web-site for more suggestions for plants hardy in your area.
If all fails you can always plant moss between your stones. Find some local moss that you like, put it in a blender with some buttermilk or plain yogourt, blend very briefly and pour the mix into the cracks.