"What do you do in your spare time?"since it tells you a lot about what the candidate is passionate about.
Now, I have been asking this sort of question in social settings because I am not a great smalltalker -- although I'd like to learn one day ;-) -- and usually asking that question (or what's your passion or hobby) get's people engaged and open up. Unfortunately some people are very obsessional and will launch into long monologues at the sniff of such an invitation, in which case it's time for me to interrupt, excuse myself and go and get another drink (especially in this heat).
Anyway, while I think it's an interesting question, I am not sure that it is that easy to interpret the answers. I would suggest supplementing it with a follow-up when there is no obvious connection:
"What have you learned from full contact macrame [or whatever] that relates to your day-job?"Let the interviewee do the hard work!