Some suggestions -
London - West end musicals, Chinatown, Piccadilly Circus, Trafalgar Square, Strand, Oxford Street (check out Selfridges exotic fruits section where they sell our local Asian fruits for exotic prices)...
Amsterdam - You should make a slight detour to Keukenhof (the array of flowers is pretty mindblowing)...
http://www.keukenhof.nl/
Paris - Eiffel tower
1) Go up top of Notre Dame towards evening time and watch Eiffel Tower slowly start to glow
2) I remembered reading Lonely PLanet book suggesting to stop a few stations away from Eiffel Tower itself. Then take a leisure stroll and take pics of the gorgeous monument. Start your journey late afternoon and capture the same effect as point (1) above
I can't seem to google the details of this walk though, so you'll need to get the Lonely planet book.
3) Walk up Eiffel tower instead of taking the lift...
Best time to go: Probably summer as you'll have more daylight to look see look see...