What are some good beginner plants for an orchid terrarium?
There are actually quite a few candidates depending on the conditions you wish to provide. There are a multitude of very small to true miniatures in the orchid family. You just have a bit of research ahead of you.
*For cool to intermediate temperatures: Masdevalias, Draculas, Dendrobiums, and Restrepia are some examples. *For warm temps: some mini-Cattleyas, Dendrobiums, and Epidendrums/Encyclia would be some examples.