Buckingham Palace

13 Buckingham Palace Rd Westminster London SW10W 0

Tube : St James Park or Victoria

Borough : Westminster

http://www.royalcollection.org.uk/

http://www.royal.gov.uk

You can visit the castle end of July to early October each year. Keep your tickets, once stamped you can reuse them during the season (for free!)

Tower Bridge

Tube : Tower Hill
Borough : Tower Hamlets


http://www.towerbridge.org.uk/TBE/EN/


You can also see the bridge pretty close cruising on the Thames.

St Paul's cathedral

The Chapterhouse, St Paul's Churchyard EC4M 8AD

Tube : St Paul
Borough : City
                          
http://www.stpauls.co.uk/
http://en.wikipedia.org/wiki/St_Paul's_Cathedral

Tower of London

EC3N 4 - 0844 482 7777

Tube : Tower Hill
Borough : Tower Hamlets

http://www.hrp.org.uk/toweroflondon/

A quick visit would last at least 2h30. Special programme from time to time.
If weather permitted there are benches along the river Thames to get a nice view on the Tower Bridge also (Starbucks coffee with bathrooms nearby!)

London Eye

Riverside Building - County Hall - Westminster Bridge Rd London SE1 7PB -
0870 990 8881

Tube : Waterloo
Borough : Lambeth

http://www.londoneye.com/

Book online to get 10% off - Long queue during pick season...
Visit last about 30minutes once in the capsule
Your admission ticket include a London Eye 4D cinema experience - on the day of your London Eye tour.

Note that you can get combination tickets also (see website above)