London Layover

I am going to Doha Qatar for Christmas to visit my parents. I fly back on Saturday December 29, with a fairly long layover in London. I land in Heathrow at 6:30am and take off at 5:00 that night. I have been to London before on a layover, and saw Big Ben and Buckingham Palace.

So my question to everyone who has been to London: what should I do with the little bit of time I have? I would like to make it back to the airport with two hours to spare.