A good symbol for Aunt Alexandra would be a magnolia tree. Like her, they are formidable and difficult to move. They are also ubiquitous throughout the South, including Alabama. In addition, a magnolia could be seen as representing tradition going back many generations. Aunt Alexandra also has old-fashioned views of how young ladies should behave. This is a constant source of frustration to Scout. Magnolia flowers are beautiful and delicate, but when they die they fall and make piles of ugly brown petals and stems, representing Aunt Alexandra's personality. She can be motherly and ladylike, but she also shifts and becomes sharp and critical.
A mockingbird could potentially be used as a symbol for many characters in the book, Tom Robinson and Boo Radley in particular. It could be used for Jem and Scout as well, but I wouldn't suggest it; there are too many other characters for whom the symbol works better.
Jem and Scout are like squirrels; they gather bits of knowledge about the world throughout the book, then hide them away till they are older. Atticus tells Scout that Jem is doing this because he has difficulty dealing with the jury's guilty verdict for an obviously innocent man. Also, they find the treasures that Boo Radley has squirreled away in the knot of a tree.