Succulents are super low maintenance and the scale is small - just right for a terrarium.
Hi Sabina: Something that is a slow grower, like a cactus, but be sure that all the plants have the same needs. You don't want a water lover in with a cactus. Here is a site that will give you some ideas about what to plant: https://www.simplemost.com/best-plants-to-grow-in-your-terrarium/ Good luck
Mostly any herb that you like will grow in a terrarium. Plus they go better in recipes and soups than Succulents and Cactus varieties.
I love the look of a venus fly trap in a terrarium!
I would go with either cactus or succulent, not both in the same one because of differing water needs. I have a succulent garden in my son's critter cage that he no longer uses and it works great! Very little maintenance once it is finished and growing. You can also use air plants in a small terrarium, they are really easy to care for, a shot of mist to each and done. Make sure what you plant will not be a rapid grower that will grow out of the terrarium size quickly.
go to you local nursery they have plenty of plants to use and can give sugestions