Arc de Triomphe. It looks like a Roman Arc. It is indeed. Read more at Wikipedia for more details: Arc de Triomphe.
Go to the top. You have to pay, but if you have Paris Card, it is included. Keep your visit to Arc de Triomphe as the last one, it closes very late. This allows you to maximize your Paris Card. Its locations is great, even in late evening. The metro is just at the corner. So no worry.
You can enjoy many things from the top, but Parisian traffic is sure fun to watch from the top. Arc de Triomphe was meant to be the intersection of the ten roads in Paris. A great rotunda, indeed. The problem is that it was build when human rode motorless vehicles (i.e. carriage and horses). Surely one will not build intersection for automobiles… at least not for 10 main roads.

What was so fun with watching traffic. If you look closely at the last picture, you will wonder why Parisian can survive such traffic. It looks like at any time you can expect accident. In fact, this is the place where cars are the most prone to accident in Paris. Unfortunately, tourists are number one cause of the accidents. (see picture to understand the rule of the road). If you plan to come to Paris and will rent a car, read well the fine print. You will be surprise to find that your insurance may not cover accident at Arc de Triomphe.
Of course you can see more than just Parisian traffic. You can see Eiffel from here. But WHY would you do so?
