Be aware of employer expectations…
Some employers are only collecting resumes; some may not be able to accept any resumes and may ask you to submit application materials online. Some are building/assessing an applicant pool; some are ready to fill specific vacancies.
Some will expect you to express your interest by already knowing about their organization; some will view this day as a career fair/information gathering process. Most employers will expect you will follow up if you are interested. All employers expect you to be professional, willing to initiate conversation and able to highlight your strengths.