Uni open day - what questions are important to ask?

Inquire about course opportunities, like can you spend a semester abroad etc

If you're going to live on campus I'd recommend looking at accommodations- ask questions about rent etc. Also pay attention to your course. Is the content something you want to study specifically? A degree in something like Biology will vary from uni to uni so make sure you're going somewhere the content is right for you. Also look into specialisms- if you want to be a clinical psychologist, or perhaps a primary teacher with specialism in science- it's important, if it's applicable to you, to see if you have the opportunity to hone your skills within your degree or whether it's just a broad degree. Things like term dates, hours of class, qualifications (and leniency in qualifications) may also be something that influence your decision.