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).
There is a unqiue eway to guage the age of a pound coin. The portrait of the Queen on it shows her at whatever her age was at the time it was minted.