A fabulous, inexpensive lunch spot atop tombstones in the crypt of St. Martin-in-the-Fields church on Trafalgar Square
Do not buy point-to-point tickets. What you want is an Oyster Card (possibly with a Travelcard transit pass).
Harrods of London installed a novelty in 1898: the world's first true escalator (to be fair, an inclined moving belt with metal bars for traction did make its debut two years earlier on Coney Island). The oddness of a moving staircase so unnerved many shoppers that employees were stationed near it with smelling salts and cognac to help revive those overcome with fear.