River Thames, London.

The River Thames, the most famous river in the UK, is known by many as the river which runs through London and is home to sites such as London Bridge, Tower Bridge, the Houses of Parliament and the Tower of London. It was also the venue for Queen Elizabeth's River Pageant, which celebrated her Diamond Jubilee. 
You can walk along the river in the Westminster and Southwark areas and see sites such as those already mentioned (Houses of Parliament, etc.), as well as some other amazing buildings and attractions. You can also visit shops, cafes and restaurants along the way. It is also possible to go sailing, rowing, skiffing and kayaking along the river, as well as go on a boat tour or hire a narrowboat. You can also visit the old warship HMS Belfast.
The river also runs through Oxford, Windsor and Eton, which have a lot of history and sites to see if you want to escape the busy streets of London for a bit.

For more ideas, click the links below:

London Eye

City Hall

The Shard