You may have more choice than you think; often kids here get into the out of catchment schools. What to ask depends what would matter to your child, what they would like to know about their future experience, which is usually more about "Where do I spend lunchtime on wet days, will I see my friends?" rather than "how many GCSEs scheduled when." Definitely make it about practical matters (strategy now & attitude towards what strategy) because things like setting strategy could easily change completely in 2-3 yrs time. Some ideas:
How do they handle bullying?
How do they communicate with parents? Parent portal, Facebook, text, email? Do you parents have emails for teacher (I love this, though I imagine poor teachers shouldn't). What about form tutor emails?
Are there any plans to change uniform soon? Ask other parents how strict the uniform policy is, though. Huge HooHaw at DS school about uniform policy right now.
The organisation of the open evening seems to reflect organisation of the school, in my experience. Especially, how well are the kids organised as helpers.
What opportunities are there for kids to take part in school life? They should mention sports, helping at open eve or parents eve or induction eve, school play productions, school council, inter school competitions?