Sometimes it can be hard to find information for those who are in the Southern Hemisphere. Here's a link to a great site that belongs to a dear friend and HPS in Australia that I think you might find useful: http://www.applegroveonline.com/default.asp
Imbolc, as I'm sure you've read, is the beginning of Spring. It is the time when the Goddess appears to us as the Maiden and the God is the Young God. It is a time of new beginnings. The word Imbolc comes from the Gaelic meaning "first milk" as it was when the ewes would begin dropping their lambs. And it was also important because it was the Sabbat by which our ancestors could see the promise of renewal at Yule being fulfilled in the greening earth and the swelling bellies of their cattle and sheep.
Because of the association between Imbolc and milk, I always try to include some sort of milk-based drink or dish such as custard in my Imbolc celebration.
Imbolc is also traditionally associated with the Goddess, Brighid (pronounced Breed). Making corn dollies or Brighid's crosses to hang in your windows is a traditional way of celebrating the holiday. One folk tradition was to make two corn dollies, one male and one female, and lay them in a basket or Bridghid's Bed to promote fertility and growth.
Imbolc is also associated with the feast of Candlemas. Lighting lots of candles to signify the growing power of the Sun is another traditional method of celebrating the Sabbat.
But I think your idea of planting an herb garden also fits in well with the meaning of the Sabbat as well.