Among the easiest in the world! The metro can get you anywhere in the city with just one or two transfers at most. Or if you want a free tour of the city while in transit, take the bus. I hop on a route just to keep my eyes peeled for a new restaurant or boutique to check out later.
Get a Navigo pass and hop on and off as often as you like.
Also, if you have a European bank account, you can use your debit card to rent a Velib (that's a bike). When the weather is nice, there is no better way to travel. Just watch out for Parisian drivers! It can be a real adventure! over 6 years ago