Ile de la CiteApprox. Time: 1 hours
Activity Level: Easiest

The Ile de la Cité is the most historical location in all of Paris because it was here that Paris was first founded, settling around 250 BC. The entire island is loaded with historical buildings. The many sites of Ile de la Cite are:
Pont Neuf - the bridge at the northwestern tip of the island
Place Dauphine- a very old square surrounded by some of the oldest buildings in the city.
The Concierge - A prison during the Revolution and is now open for tours.
The Palais de Justice - This is where the courts and the heart of the French justice reside.
Marché aux Fleurs et Oiseux - an amazingly big flower market and you can find just about everything here.
Hotel Dieu - a city hospital, one of the oldest and largest.
Notre Dame - the most well known cathedral in France.

Most places on the Ile de la Cité can be seen quickly because they are all so close together, even though there are so many. One bonus it to cross the Seine to the south and walk along the quais down by the river. You get some incredible views of the island and Notre Dame. Be sure to take a visit to this island.