Velib is Paris' bike sharing program, and not only is it really fun riding around Paris, it's a pretty sweet deal. For 1.70€, you get 24 hours worth of bike rentals, which means you can go to any Velib stand in the city (they claim they're every 300 meters, and I wouldn't be surprised--they're everywhere) and then return it at any other Velib stand. For each individual trip, if you return it within 30 minutes, there's no additional charge. (Which means on longer trips, we could find a Velib stand, return the bike, and get another one out immediately, and not get charged!)
We ended up Velibing all over the city, from our apartment in the Marais to the Eiffel Tower on one side of town and up to the Sacre Coeur on the other (though down from the Sacre Coeur was a much more fun ride...), and everywhere in between. There are even bikes lanes a lot of the way!
The only downsides we found were that 1. You had to be really careful to get a bike that actually worked (we accidentally got bikes with no brakes, one with no chain, and one missing a pedal over our time there). Luckily, if you get a dud, you can just return it and get a new one. And 2. These suckers are popular. Even though there are tons of stands, we'd occasionally have to visit two or three or six stands until we found one with two working bikes available.