Hermione Granger is a popular choice for a Halloween costume. She's the main female character from the Harry Potter series so the top choice. (If you're looking for other female characters from Harry Potter for Halloween Luna Lovegood, Ginny or Molly Weasley are good choices too.)
When creating your Halloween look a Hogwart's gown is the obvious choice for your Hermione costume. But if you don't add something special to this standard costume you're just another 'run-of-the-mill" Hermione character for Halloween. One of the many that will be out and about on Halloween. So how can you make your Hermione stand out from the others?
Accessories matter is a rule that applies to costumes as well as fashion. By picking the right accessories you can make your interpretation of Hermione stand out from the crowd. Your accessories may be something you find at home. A couple of big books (since Hermione had such an addiction to reading) is one idea. Although books might become rather heavy and awkward to carry around on Halloween.
There's also some great accessories you can buy. Some are more obvious choices than others. The all essential wand and optional wig are in this category. Some accessories are only really suited to Halloween (like the wig) while others can be kept long after the holiday. Great ones to keep are the time turner, Crookshanks and Hermione's bag.
Which accessory you decide to use all depends on what image of Hermione you want to create for Halloween.