While it may be debatable, I think the best type of plant in any outdoor enclosure is fountain grass!!! The stuff grows fast, gets big, provides a lot of shade, and looks really good too. It can be eaten, a few of my tortoises seem to really like it, but I think for the most part tortoises won't eat it.
Here is one of our hides surrounded by 4 fountain grass plants.
Here is one of our hides surrounded by 4 fountain grass plants.