I haven't really thought about it, but things that are cheap, hardy and easy to grow. I noticed this post http://www.fnzas.org.nz/fishroom/viewtopic.php?p=708564#p708564 the other day which suggests java fern, java moss, xmas moss, hygrophila polysperma, crypts and indian fern.
I'll certainly consider it. How much would postage to Palmerston North be?