Pick weaknesses that are not a core skill for this position. You can be candid in your answer; recognizing that you aren't great at something and acknowledging your need to improve. Be sure to have an action plan in place for improving on this weakness.
Perhaps you are watching TED talks to gain skills in a particular area, reading the latest-and-greatest book on the subject, or maybe you are taking a seminar at a nearby community center. We are all human with our weaknesses, so don't be afraid to share yours!
"I believe I could improve on some technical skills including Excel and PowerPoint. Currently I am at a beginner to intermediate level; however, I would be more comfortable at an advanced level. I have enrolled myself in an evening/weekend workshop for the next six weeks. We will see how stellar my skills are after I complete that course!"