You didn't mention whether your terrarium is open on top (to permit air circulation) or if it is closed. If it's closed, African violets won't do well, because they like good air circulation and they dislike having water dripping onto their hairy leaves. Many foliage plants thrive in both open and closed terrarium environments. Suitable choices include aluminum plant (Pilea cadieri), artillery plant (Pilea microphylla), and creeping fig (Ficus pumila). Ferns, such as bird's-nest fern (Asplenium nidus) or table fern (Pteris cretica), also grow well in a terrarium. Nerve plant (Fittonia) adds colorful leaves to your planting blend. All of these plants are easy to find at garden centers.
Consider using miniature versions, which won't outgrow their contained environs as quickly as their full-size cousins will. Most terrarium plants die as a result of overwatering. If you keep your container closed, you shouldn't need to add water after the initial planting. For an open terrarium, add water only when the soil is dry to the touch half an inch below the surface.