I went with tropical plants.
The terrarium is designed to keep the humidity in. I could have done cactus, but the lid would have to be off most of the time.
I put a large piece of wood in the background, then built up the dirt behind it.
Added some rocks and angled them in different directions, hoping that would give it a more natural look.
I also added one small mushroom miniature. Just one... toward the front and slightly tilted.
In this picture, you can see that the Buddha is holding a bowl. I've got some of those battery operated, fake little vigil lights. It fits perfectly in the bowl. It is awesome when it's a little cloudy outside. (like this morning) I turned on the little light, and oooh...what a serene sight it was.