Without a doubt my favourite thing in all of Paris was the Velib system. The velib system is a city bike system, where for €8 per week (or less if you are there longer term) you can pick up a bike from one of the stations all over the city and ride anywhere you want (so long as you drop it at another station within 30mins).
It was so much fun being able to cycle around the city and take it all in, with a really easy method of transport, and being able to get around so many different places so quickly. Granted there were sometimes some hiccups, like dud bikes, or being charged extra for going over time, or not being able to find a bike nearby when leaving a popular place at a busy time, but overall it was a fantastic system and I really loved it!
If there’s one thing I can definitely recommend in Paris it is grabbing a bike and cycling round to take it all in!
I wish it was that easy in more cities (many you have to live there or can only get a 3 month membership etc)