I presume this is without water???
If so then I would recommend any or all of the varieties of Fittonia for a start. I believe these are in the Acanthaceae but commonly known as the Acanthus Family.
They are generally low growing plants that sprawl and that can root at the nodes where leaves are attached to the stem of the plant. So they can make a very nice cover in a terrarium. They like moderate light and will do well in shade, from 300 to 800 foot candles, probably handle higher . There are only two species (one is not common in cultivation while the other, F. verschaffeltii consists of three distinct varieties that are often sold as species) and while the leaves tend to be similar in shape and venation the size of the leaves and the color of the leaves vary adding a nice palette to a terrarium as an en mass planting.
Here is a link to images of Fittonia:
click here>
http://aolsearch.aol.com/aol/image?q...nType=imageTab