what makes a good university (or department), in your experience?

I'm planning to go to universities' open days (physics departments) and I would greatly appreciate it if someone could please tell me what I should look for and what questions I should ask. I know it might sound somewhat silly - asking you to tell me about what I should ask - but you've already had an experience at uni so you should know what the good things and the bad things are. I can give a full review of what makes a good sixth form college - but i'm clueless when it comes to unis because I've never been!

Thanks for your help!