I love moss so that's what I'll go with. You'd be surprised how interesting it is. Again these are the items you will need, plus spring water.
OK, dirt requirements are different for each plant. Moss won't need more than 1/2" to an inch. My orchid needed the height of it's existing container. This is also where you'll need to make sure you have enough head space for your new plant in it's jar. The carnivorous plants only needed sphagnum moss. Find out what your chosen plants needs are.
Here's a situation that I won't be using sphagnum moss. Like I said, I use it with my pitcher plants and when I can see the dirt between plants. I have enough green moss to cover the dirt this time.
My pitcher plants have been around for years and 2 of my jars have volunteer ferns growing in them. They are so easy to keep and enjoy. About twice a year I'll remove dead leaves and check water height. These plants also require filtered light. Full sun can really heat up a terrarium and kill your plants.
This is when you need to give your terrarium water. Use spring water and fill to the top of the rocks. You don't want your dirt water logged.