Actually it all also depends upon which kind of industry you belong to and what general background your CEO has, but still below are some inputs from my end-
1) Which one would you say is the most important ability a leader should has?
2) How will you handle the situations where you have to make decisions against business ethics?
3) What do you think is really at the heart of lack of the teamwork?
4) How do you start your ordinary day? How does it end? Do you still have enough time to sleep and take a rest?
Would you like to ask questions for the sake of it?
In open house sessions, issues are raised for which your managers also do not have answers. Questions should be specific to your business or company. We in the public forum cannot ascertain your company's issues.
On a general note, I would say that you may raise questions on the business plan of the company, strategic plan of the company, launching new products, entering into new geographical area, internal company issues etc. However, you need to take into account the psychology of the person also. He should not get annoyed because of your questions.
This is a good step by CEO and shows his willingness to interact with his employees.
While I second what Mr. Divekar mentioned that CEO should not be annoyed with the questions, it is a good idea to envisage what he thinks about his employees, overall business and what are the future plans, succession planning etc. As written above, it also depends on his personality how he will take it if he has to answer usual questions. This is more advisable over informal meetings where CEO wants to interact on one to one basis.