The London Eye was nice.
However I enjoyed Piccadilly Circus, Westminister Abbey, the Victoria and Albert Museum, etc.
Paris it is quite obvious that if you have the time, you should certainly go to the Louvre.
In Italy, depending on where you are going, The Coloseeum, Tower of Pisa, or if at all possible...the Vatican.
Whether religious or not, that place has some amazing history, art, architecture and you will gain a different perspective on life with its aura or peacefulness.
Regards,
Menelaos