If you fancy taking a bus to see the sites and are looking to cut costs, then you can take a regular service (free with a Travelcard) which passes a number of significant sites. Okay - no commentary, but it's cheap.

Two routes are good contenders:

RV1 Bus

Tower Gateway Station -  Tower Bridge Road - Tooley Street - Duke Street Hill - London Bridge Station - Borough High Street - Southwark Street - Tate Modern - Blackfriars Road - National Theatre - Waterloo Station - Waterloo Road - Waterloo Bridge - Aldwych  - Russell Street - Covent Garden

23 Bus

Liverpool Street Station - Threadneedle Street - Bank Station - Queen Victoria Street - Mansion House Station - Queen Victoria Street - Cannon Street - St. Paul's Churchyard - Ludgate Hill - Fleet Street - Strand - Aldwych - Strand - Charing Cross Station - Trafalgar Square -  Pall Mall - Waterloo Place - Regent Street - Piccadilly Circus Station - Regent Street - Oxford Circus Station - Oxford Street - Bond Street Station - Marble Arch Station - Park Lane - Cumberland Gate - Edgware Road  - Paddington Station - Westbourne Grove - Ladbroke Grove - Canal Way  (Little Venice )- Westbourne Park Station

| More...