This one's fairly self explanatory: go to London and make your way to the nearest stop on the circle line. Find a nearby pub, have a drink, and travel to the next stop, have a drink... until you return to where you started. This is not good for your health, even if you're drinking halves.

Some people who take this all very seriously have devised complicated sets of rules for the pub crawl. Not wishing to plagarise, you can find one such example at: http://www.roundel.demon.co.uk/circle/