Admitted Students days can be exhausting but informative, in the best case scenario. In all of my experiences, I attended these hosted visit days after already accepting my admission; however, many students go hoping to find another tidbit of information that can help them with a difficult decision. Regardless which camp you fall in, you should find this information useful.
Prep work. Since you've already applied to the school, you're likely pretty familiar with their program. But as you consider actually going there, you may have more nitty gritty questions related to funding, coursework, or general program structure. These are the hard pieces of information you should use to compare and contrast different schools. Next, think about your past educational experiences. Do you do better in large lectures with separate teaching sections, or in small group discussion classes? Do you utilize office hours like it's your job or do you prefer study groups with colleagues?