What kind of plant can I fit in here?

Creeping thyme or elfin thyme would be pretty. Also cat grass or sweet violets.

Wheat or rye grass will grow, but only for so long before the roots get too long. If you want to keep changing it out, you could start any kind of plant seed. Small creeping succulents like sedum will grow for a long period of time and provide color. Dragon's Blood Sedum is one of these, with burgundy foliage.

I think theres a plant called Glun that can fit in there its very small but the roots are some what long so i thing u can fit a nice Glun in that little pot.

May have to go with miniture cactus. Herbs (rosemary, thyme, mint) don't need lots of root space but they would be too tall & tip it over but chives would work. Black seeded leaf lettuce would work since it grows straighter than an herb and if you pinch off the leaves to use before plant gets too tall.

Miniature cactus or chives & possibly leaf lettuce.

I would suggest to plant small cacti, why? Because the box is small and it has no drainage. So cacti are the best plants because they don't require any watering if they are left indroors. I like your idea because I like recycling

stonecrop or hen/chicken type succulents. If you can get a small Jade plant (or start one from a cutting ) you could grow it as a bonsai. Also, you might look for a small bromeliad, fern or other epihytes.

Try an "Air Fern". I think it would be beautiful and is very easy to grow.

Hope this helps..

a small succulent or an herb.

thats way cute. they have tiny cacti