I asked a lot of questions related to teaching and TAing. It's important to know when you'll be expected to actually teach in front of a class (one school said first semester!), whether you will be given instruction (such as a teaching effectiveness seminar) before teaching, and what kinds of teaching opportunities you will be given after you have a master's degree. Some programs allow you to essentially teach your own course, if you're interested in doing so.
You can ask faculty or students what they like and dislike about living in the area. You can also ask students (preferably without faculty there) if they can live comfortably with the stipend, whether or not most people have roommates, and which neighborhoods are the best for grad students to live in. Ask them about their office spaces too. When there are no faculty around, you can ask the students what they think of their coursework and your POI's mentoring style. Throughout your time with them, pay attention to how well they appear to work together and whether or not they essentially like each other. It's important!
In a group meeting, you can ask whether or not the school offers health insurance. You might also want to know their procedure for qualifying/candidacy exams, if they don't go over it with you.
For the person who asked about the individual interview with the POI-- yes, definitely have plenty of questions prepared. You can ask what upcoming projects they will be working on. How many students are currently in their lab. How long students usually take to complete the program. I've done two of my interview weekends so far. The one that was a good fit was easy, and we kept talking until we ran out of time. The other one was more awkward, but I got through it. You'll be surprised at how quickly you can think of questions when necessary! But if it's a good fit, you probably won't need to fire off question after question.