Last minute sightseeing to fill a day and half in London

Assuming you don't mean that part of London which is called the "City of London":

British Museum, Greenwich Observatory, National Maritime Museum, St Pauls, Westminster.

The hop on/off bus gives a good overview. Go all around once, then get off where you are interested the 2nd time around. The Tower is well worth some time. Covent Garden, Camden Locks market and Leciester Square are good people watching places. Kew Gardens is lovely almost anytime of year.

A lot of the major museums such as the British Museum, the National Gallery and the Victoria and Albert are free. At Westminster Abbey and St Paul's you have to pay admission, but I think they are worth it.

My top list for London short stay - Westminster Abbey, St. Pauls (climb the dome steps), National Gallery (focus on Rick's tour and the Impressionists), British Museum (focus on antiquities behind the Rosetta Stone). Museum and Gallery are free.

Best sections of town to stroll, people watch and get cheap food - Leicester Square, Covent Garden.

Night activity cheap - concert at St. Martins in the Fields church (cafeteria in the crypt) - on Trafalgar Square. Night activity costly but worth it - show on the west end or evening ride on the London Eye.

Not worth the time and energy if you only have a day - Buckingham palace changing of the guard, though the palace is worth a look. Harrods. Oxford Street.

We enjoyed the Imperial War Museum and it was free.

Westminster Abbey, Tower of London, St. Paul's Cathedral, Churchill Museum and Cabinet War Rooms are important sights that are worth a fairly high admission fee. Free attractions include
*British Museum,
Museum of London,
Sir John Soane's Museum,
*British Library,
Geffrye Museum,
Bank of England Museum,
Museum of Childhood,
Wellcome Collection,
Horniman Museum,

Petrie Museum,
National Army Museum,
National Maritime Museum,
Royal Observatory,
*Imperial War Museum,
Royal Air Force Museum,
Hunterian Museum,
Guildhall Clockmakers' Museum,

*Victoria & Albert Museum,
Science Museum,
*Natural History Museum,
Tate Modern,
Tate Britain,
*National Gallery,
National Portrait Gallery,
The Wallace Collection,
Changing of the Guard,
Courtauld Gallery (Mondays from 10.00 until 14.00.

Asterisks denote my personal rankings. Visit the places that interest you.