The London Eye

The London Eye is probably the most famous of London's attractions by now, perhaps because it's so big, towering above the old Greater London Council building opposite the Houses of Parliament. Originally it was comissioned as the Millenium Wheel, to be present for one year only but then again so was the Eiffel tower in Paris.
Trips on the London Eye can be pre booked with London Theatre breaks which is a good idea if you're on a busy weekend theatregoing schedule as well as taking in the sights,
but otherwise it is possible to queue up and get a ride on the London Eye the same afternoon if it's a quiet day.