Well, the obvious answer is, "Ask about things that you're truly interested in!"
But I'll give you more than that. Your questions should 1) demonstrate a good amount of knowledge about the program and 2) show your enthusiasm for the program. But don't try to get too clever and ask very specific things (like "Can I get into Professor Smith's macroeconomics class? I love his work.") just to try and impress adcomms. They'll probably be annoyed by this.
You definitely don't want to come in with an "You need to impress ME" attitude, but it's very reasonable to ask about things like the what the career office offers and faculty accessibility. Or you can ask about curriculum flexibility, how many majors people take on, etc. But hopefully you're not asking basic things that can be read in brochures.
Finally, LISTEN to their answers! The worst thing you can do is tune out and think about your next clever question while they give you valuable information.
I hope that helps. I'm reluctant to give you specific questions since those really depend on you and the school.
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