You've probably seen it before, bicycles for rent from a machine, randomly setup around a city. It's not unique to Paris, but its a cheap, quick, and cool way to find your way around the city. You'll need to buy a ticket for the day for 1€, then trips less than 30 minutes are free, the next half hour is 1€, the following half hour is 2€, and from then on it costs 4€ per 30 minutes. You're then free to bike through Paris for as long as you like, and drop it off at any other station when you're done. Be warned that if your credit card doesn't use the chip and pin system, you won't be able to use it for Velib.