A London trip is always something special. You wanna see the highlights of this awesome British capital, right?
In order to get a full breeze of the historical sites you need to take a boat tour on the Thames.
My recommendation: Take the KPMG Thames Clippers with a day ticket and sail back and forth from Embankment via London Eye to the O2.
With a valid Oyster Card (cost you GBP 29.50 for unlimited rides on underground, overground, DLR and buses in zone 1-2 for a week) the ship cruise is 1/3 off the regular price; for an adult, it's GBP 9.10.
OK, if you stay less than a week take the Oyster Card anyhow. You will surely ride more than 10 times on subway trains or buses, unless you are a passionate wanderer and painless after hourlong walks.