Thanks to Baron Haussmann's mid-19th-century redesign, the City of Light is a compact wonder of wide boulevards, gracious parks, and leafy squares; and, without question, the best way to explore it is on foot.
When you want a lift, though, public transportation is easy and inexpensive. The métro (subway) will get you just about anywhere you want to go for €1.70 a ride (a carnet, or "pack" of 10 tickets, is €13.30); tickets also work on buses, trams, and the RER train line within Paris.
If you’re sticking to sites on or near the river, Batobus offers a convenient on-the-water alternative. A ticket for one day of unlimited hop-on/hop-off boat travel costs €16.
Paris is divided into 20 arrondissements (neighborhoods) spiraling out from the center of the city. The numbers reveal the neighborhood's location and its age, the 1er arrondissement at the city's heart being the oldest. The arrondissements in central Paris—the 1er to 8e—are the most visited.
It's worth picking up a copy of Paris Pratique Par Arrondissement, the essential map guide, available at newsstands and bookstores.