Succulent

If I had enough space in my current living situation, I would definitely consider making a hanging succulent garden.  They take up very little space because you can hang them up, and don't require a lot of care after planting them.  You could probably move them indoors or outdoors if you plant them small enough to transport.  Plus they look super pretty!

If you want to get more creative with it, you can choose interesting shapes for the plants to go in.  This is an example from artist Judy Kameon for the New York Times.