Not only is the London Eye beautiful to look at from afar, a ride in one of its capsules offers amazing views and a rather thrilling experience. I went at night, and it was wonderful to see the city lit up. Quite romantic, too.
The views of Parliament were the best, but there are plenty of other recognizable sights, like St. Paul's Cathedral and the Tate Modern. And the capsules are quite comfortable, since they're not overstuffed and there is plenty of bench and window space.
You can rent out a capsule for a private party, and if the line seems extra long, you can also buy a sort of fast pass that'll get you to the front of the line for a few extra quid. When I went on a January evening, though, we practically walked on with a general admission ticket, which was only £15 a person. I think we waited about 5 minutes.