What is the best media/substrate [soil] for a terrarium?
It depends on the plants you intend to use. For bog plants a mix of peat moss and perlite is often a good choice. For cacti, the cacti mixes sold in stores works well. For most house plants, standard potting mix may be used. Vermiculite, bark chips, coconut husk chips, and some orchid mixes are also suitable. Do your research and feel free to experiment. Of course please let us all know what you've found worked for you! : )