The possibilities are almost endless, but what you grow depends on what kind of lighting you can provide. I'm using a 10 gal aquarium right now with an open top and have 20 plants in there and still room to spare. You can have potted plants in the bottom and mounted plants hanging from the sides, so it's really possible to grow a lot in a small space.

Here's a few that have done exceptionally well for me:

Any of the Restrepia - the flowers are like little aliens and just cute as can be.

Pleurothallis picta is another cutie and blooms several times a year with dozens and dozens of tiny, brilliant yellow flowers.

Aerangis mystacidii - is mounted and hanging from the side.
White flowers.

Maxillaria sophronitis - is mounted and also hanging from the side. It has brilliant orange flowers

Sophronitis cernua - might be a possibility depending on how much light you can give it. Blooms several times per year or more and again, brilliant orange flowers. Absolutely adorable!

Check out Lynn O'Shaughnessy for the Restrepia and Pleuros.
Link below.

Also check Oak Hill and J & L Orchids.

Kevin

Here is a link that might be useful: Lynn

Dendrobium jenkinisii _creeping, mat forming plant with spring orange flowers smells sweet like honey.

Dendrobium abberans - dwarf species from higher elevations of New Guinea. Blooms with small white flowers. Really nice species for terrariums.

Cattleya luteola - Small species that doesnt grow much more than 4-5" with yellow flowers diffused with red in the lip. Needs bright light, and doesnt like to be constantly wet.

Dockarillas might work too.

Depending on your temperatures and light many of the Masdevallias may work for you!