First of all, it looks like a really tiny terrarium (considering it's not much higher than a spray bottle...). If it is as small as it look, the first thing would be to get a proper enclosure for the frog. If you put that aside, the only thing I would add is climbing and hiding space, like some nice (real) plants (will help create a more stable environment as well). Pothos (Epipremnum aureum) is a very good example of an excellent tree frog plant.