Start off with a good birds-eye view.
For the best views in Paris, check out the terrace of the Printemps department store (much better than that of the neighbouring Galerie Lafayette!), or go straight for the monuments and climb the 300 steps up the Arc de Triomphe for a stunning panoramic view of the centre. The Eiffel Tower is without a doubt a great view, but keep in mind that you can't see the tower itself from the top! I recommend you climb the stairs - it's way easier than you'd think, and you will save yourself heaps of time waiting in the never-ending line for the elevators (more on climbing the Eiffel Tower in a future post...). I'm also assured that the view from Tour Montparnasse is stunning - that said, I haven't been up in that infamous skyscraper yet, so I can't confirm.