Check out turtlestuff.com for some great ideas.They offer a wide range of seeds that are both popular and nutritous for tortoises.

Dandelions would be first on my list followed by burmuda grass, and hibiscus plants/bushes. These are all very easy to grow, and will offer a good base for a grazing tortoise. The hibiscus will also offer shade for the hot days. Plantain is also a very good plant that grows quickly and offers high nutritional value. There is a giant variety available somewhere online, but I can't remember the name of the site off the top of my head.

-Kory