I thought I'd start a thread where people could share one or two places they came across, and share that information with others so they can check it out too.
For London - if you go to Tower Hill tube station, there's a raised area accessible using concrete steps. Doesn't look like much, looks like it's covered in ivy, but go up those steps. Not only will you get THE best picture of the Tower Of London you're ever going to get, there's a huge sundial on the floor up there and the pictures going around the circumference tells the story of London over a 2000 year timespan. From the Roman invasion, the founding of the Tower in 1066, Shakespeare arriving in London, the Great Fire, the building of the Houses of Parliament as we know them, the start of the London Underground, World War 2 bombings, and the building of the Thames Barrier.
Salisbury - there's a Waitrose supermarket close to the river Avon. Buy a single bread roll, and go feed the ducks and swans. The swans seemed to be very polite and would line up in the water for their crust of bread. Very polite birds on that day.