North Americans relying on public transit within London can do like the locals by pre-purchasing an Oyster Card. The pass makes taking buses, subways, trams and trains easier: instead of clogging up queues while you figure out fares, you just touch your electronic “smart card” to the yellow turnstile reader. It’s cheaper, too, because Oyster Cards automatically adjusts to give the lowest possible rate for all trips made within a single day. Pre-loaded cards, available through the British tourism agency’s online shop (www.visitbritaindirect.com) start at $15. If you use up your credit, simply re-fill the card at one of 3,000-plus machines in London. Since Oyster Cards never expire, leftover amounts can be saved for return visits or transferred to another traveler.