Velib, the way to go

The Mayor of Paris has established a system of bike rentals that is available about every 3 or 4 blocks. It is called Velib'

You can get an annual pass for 29 euros, or a weekly pass for 5 euros. There is a charge for using it over 30 minutes (but you could, for example, return a bicycle after 29 minutes, and immediately get another one).

More info:

http://www.velib.paris.fr/

Paris is ideal for this, as it does not have too many hills, and the dowtown area is fairly compact. On the other hand, you do have to deal with the Parisian traffic, except in those areas where there is a bike route.

Highly recommended, and good for the environment!