Eiffel Tower

Go up, and time it for sunset. The very top was shut on my night because of high winds, so I only got one level up - and I took the stairs, which as someone scared of heights I did not love! Completely worth it though: the Champ de Mars straight below you, then the sky going orange and pink and the whole city switching its lights on. I came back to see it from Trocadéro and Bir-Hakeim too, and it's different every time.