Echeveria 'Violet Queen' is a very easy going succulent! It has gorgeous greenish-purple foliage that deepens in colour as it matures. It has a compact, clumping growth habit which makes it perfect for open terrariums.
Echeveria love full sun so make sure to keep your terrarium in a bright space.
Bonus points - It's also nontoxic to pets so it's a great choice if you have curious furbabies.