When an employer asks you during an interview, "do you have any questions for us?" Here are a few things you can ask. These will not only give you insight into your potential new job, but will also show your potential employer that you're capable of asking relevant questions. Try these on for size:
1) If you were to hire me, what would be the number-one task on my list of things to do?
This is a great way to see what you'd be doing at work, beyond what the generic job description has already told you.
2) I already know that your company's strengths are -- insert company values from mission statement here -- but what would you say is the most important part of your culture as a company?
This shows your potential employer that you've familiarized yourself with the company's values and are curious about how you'll fit in.
3) Based on our interview, is there any reason you think I might NOT be the best candidate for this position?
This is a scary question, but it can be extremely helpful to everyone at the job interview. If gives your potential employer the chance to address any issues, and gives you a chance to allay any unjustified reservations they might have about you.