If you visiting London and want to get a really nice view of things on the Thames then I can recommend walking half way across the Waterloo Bridge and stopping. This gives you a great view down the Thames towards the London Eye, Houses of Parliament and a good view back the other way towards St Paul's Cathedral, Millennium Bridge and the rest South Bank.
If just gives you a good sense of where you are and lets you take a little more than the tightly packed streets of London often allow.
