What do I have to see in Europe? (see inside for specifics)

I’m headed on the trip of a lifetime soon…

For those of you who have been to Paris, London, or Amsterdam, what are must-sees? Or what restaurants must we eat at?

We’re keeping the trip fairly loose. I know we’ll do the normal tourist stuff like the Eiffel Tower, Trafalgar Square, and the Red Light District. I also want to see the Van Gough museum. I’m not sure if we’ll do the Louvre or not.

Can you suggest anything else the boy and I should see or do? We’re pretty mellow, like culture and the arts, really enjoy fine dining, and are into quirky things.

Side question: We’ve been reading travel books, but are there any unexpected cultural differences we should be aware of so we can respect local culture?
Thanks!