Let him guide the conversation in the direction he wants. I have had CEOs who only wanted to talk personal stuff at lunch and others who couldn't leave business alone for ten seconds.
What you need to do is prep both ways and be ready to do either. What you need to be careful of is that you make a friend and not a political enemy because he has much more organizational power than you do. This is a great opportunity to impress a person who has control over your salary, your benefits, and your continued employment at that company.
On the personal side, try to stay away from religion or politics or anything that he might find controversial. It is safer to talk about how much fun you had at Comic Con than how much you hate Donald Trump and find out afterwards that he just contributed $10,000 to Trump's campaign.
On the business side, be ready to talk about things that are going well and project successes you have been a part of. If he wants to talk about how your group contributes to the business, you need to sound knowledgeable about both your professional specialty and the overall business of the company. This is your chance to become very visible and it can help you greatly or harm you.
Perhaps be ready to talk about some things you think could be improvements (ones your boss would approve of, whatever you say in this situation is going straight to him after if the CEO asks you what ideas for improvement you have). If you have an idea that could open a new market for the business, that is a great thing.
Same goes if he asks specifically for anything that is not working well, pick something your boss would love to get fixed and not something he has been opposing.
You might consult your boss about things he might want brought up if the opportunity presents itself. Tell him you want to make a good impression for the whole department and ask his advice about what to say if the CEO wants to talk business. Your boss might be thrilled to have the chance to have someone informally bring up something he hasn't been able to get past the gatekeepers below the CEO.
Try not to bad mouth anyone. No matter how much you think the VP of HR is an incompetent jerk who has figuratively raped multiple employees, it won't go well for you if you actually say that. Especially if the person in question is a C-level employee who was probably doing his or her best to implement the policies the CEO wanted.
If he does ask you to suggest something that the company can make better, be prepared to make a business case for your suggestion. Tell him why it is important and what the company bottom line will get out of it or what serious problem that it will solve. It helps here if you are aware of other things going on in the company. For instance if you know that they just had trouble in an audit or certification process due to an issue, a suggestion for how to fix that is perfect. Do not suggest anything that costs money but has no apparent benefit outside the Programming department (yes we would all love to have three monitors and brand new equipment and a chance to play foozeball during the work day, unless you can make a case for how that will improve things for the external or the internal customers of your services, then now is not the time to suggest it.
Even if he doesn't buy off on the change suggested, the fact that you presented it in business terms not usually associated with development staff is going to impress him.