There are a number of small trees that do well in containers. I don't know where you are or what your climate is. Some general suggestions for container trees include: dwarf alberta spruce, holly, japanese maple, vine maple, dwarf citrus, boxwood, crepe myrtle, redbud. But you need to find trees that grow in your area. A number of regular trees are available in dwarf varieties, so look for trees that thrive where you are and then look for dwarf versions.
Here's a nice little article on growing trees in containers: http://extension.oregonstate.edu/news/story.php?S_No=4&storyType=garden how to grow a potted tree