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?
Questions to ask. When you talk to current students, ask them how the course load is and what the professors are like. Do they do research on the side? Work a part-time job? Get a feel for what being a student in this program is like. Ask about the academic support of the department. These are the staff you've probably already interacted with to attend this admitted students day. You should be able to tell if a student is hesitant to "bad-mouth" an ineffective departmental structure, or if you have nothing to worry about. What I mean by academic support is the portion of the department that organizes materials, schedules key meetings, and communicates program requirements and deadlines to students and faculty alike. Being clear on what you need to do and by when is incredibly important, and the added stress of an inefficient support system will wreak unnecessary havoc on your stress levels. Finally, ask the student who their advisor/mentor is, and how often they interact.
When you talk to faculty, ask them about their professional goals. It's a different take on "what's your research" and may even make them pause, as they expected to give their typical canned response. Ask them about their advisee group - do students meet together with the advisor, one-on-one? How often? What does the professor expect from advisees? You'll get a feel for their interaction style and gauge their interest in mentoring.
And obviously don't forget to ask about funding.
Reflection. Looking back through your admissions folder, think about the presentations you attended and the people you talked to. What were the overarching themes they were trying to pitch you? That they are the premier institution for XYZ? That they are student-centered and supportive? That they are doing research all over the world, and here at home? That they are committed to causes ABC? You'll get more of a gut feeling about this, but I can tell you from experience that two universities can be incredibly successful research institutions but provide completely different learning environments. Think about if you thrive on the challenge and struggle of a cutthroat program, because when you get to the top you know how much you overcame...or if you flourish in a supportive environment where you're treated like a teammate, not a serf.
Good luck! Hit me up with any specific questions you have.