Job or Career fairs never indicate cover letters only resumes. I don't see how this is beneficial but I recommend you work on your 'elevator pitch' to make the best impression.

I agree with Samantha's posting below. Definitely try and keep it generic but still include enough information about yourself and highlight several of your career goals. I would advise sharing something unique about yourself within the cover letter so that yours stands out from others.

Keep it generic and address it to “To Whom It May Concern”. Mention the Job Fair information your attending because you might get extra points for participating in the job fair as opposed to just applying online. List specific accomplishments and career opportunities that you think best match your level of expertise and experience. Highlight your skills and try to keep it as short and sweet as possible.