Not really a secret spot; but after visiting Paris a number of times, Montparnasse was a complete surprise. It offers the the most perfect views of Paris viewed from either the 56 floor, or a short walk up a three flights to the roof. From the roof you can see a 360 degree view with no glass restricting your view. It is really wonderful. It costs between 10- euros to enter, but if you purchase a daily travel card they you get 3.5 euros discount.