The Arc is a fascinating sight. Located in the middle of Europe's largest roundabout, it is also the site of the last stage of the Tour de France every year. That alone probably makes it second in sights only to the Eiffel Tower.
The sight is familiar: pay your admission and queue up with many others to climb a couple of hundred spiraling stairs. The Arc does offered a discounted rate for students. At the top of the stairs, the view is spectacular, of course. For slightly more money, you can use one of the sets of binoculars and get a better view of certain sights.
The Arc also hosts the Tomb of the Unknown Soldier. Far fewer people visit this part of it, but it is also beautiful and moving.