Heading to Stuttgart, 9 hour layover at Heathrow tips (don't forget IOS Van)

So, I'm back from a short roadtrip vacation with Kate, and I'm just about to go to the airport to fly to Stuttgart. Only it turns out I have a 9hr layover in Heathrow tomorrow, starting 11am London time. Any tips on what do with roughly 6 hours of free time?

Also: don't forget about the Identity Open Space event here in Vancouver (check the wiki for more info), happening next week on July 20th and 21st. I am unfortunately missing it, but Colin Brumelle is attending on my behalf.