How much time have you allocated for each city? All of the major tourist attractions are major time sinks unless you get there really early in the morning or reserve a time slot online (London Eye, for example).
London
Buckingham Palace
Parliament
London Eye
Various parks
Rome
Vatican
Paris
Louvre
Eiffel Tower
Notre Dame
WikiTravel can be a great resource though it is occasionally out of date. Also check out the regional
Travel/Dining sections on FT.