Then there was the occasional one who would do what Alison mentioned with the sickness excuses, and create a tale that read like a police report: “I must miss my deadline because, on the night of August 12, my 45-year-old sister was alone in her house when an intruder entered. He was a 6’1″ caucasian male wearing a black balaclava and carrying a candlestick. As my sister approached him, with the dog barking around her heels, she heard a distant car crash which led her to have a fatal … etc.” (This is not an actual excuse I received, just similar in detail to some of those that were submitted.) These ones I was pretty sure were a writing exercise, requiring time and effort that could have been put to better use on the actual assignment they had been given.
An out of office message could become an opportunity to connect with your client on a more personal level. Consider sharing something about yourself that they might not have known about while working with you.

This is typically what I do. People don’t need to know where you are or what you’re doing. It’s really none of their business. They just need to know how long you’ll be out, that you won’t be checking email or vm while you’re gone, and who to contact instead.
Obviously, not every out-of-office is set for a vacation. You also need an out-of-office if you go on an extended business trip or to a conference. But instead of simply telling people you’re at a business event, why not use this opportunity to encourage networking of new business connections?