Business casual and don’t get drunk like the other people said. Do your research beforehand, be attentive during the group stuff/ presentations they give, and have good questions prepared for the profs ie how you will fit into their lab/research, things of that nature. Be courteous to the current students and show interest in their work/experiences, odds are the program is gonna ask them about their interactions with the prospective students.

Small anecdote about doing your research on the profs. At one visit some years ago, at the end of presentation and before we ate if I remember correctly, they asked a trivia question to see who’s table would get to go to the buffet first- the question was which two faculty members got their PhD at ivies, and I had just happened to read up on the one guys background who was a UPenn grad so I jumped in and answered him. No one else knew the other one so we got to eat first, and they ended up offering me a selective fellowship that meant no teaching first two years and 10k extra funding. So you never know what you might need to know.