Oh! Also, this is a recommendation for going anywhere in Europe really but open a Monzo account.
It's an online only bank which allows you to withdraw up to £200 abroad with no fees. Take your Euros in hard currency, whack a couple of hundred into your Monzo account for any emergencies.
Here's...
You'll want to grab yourself one of these each:
https://booking.parisinfo.com/il4-offer_i149-paris-visite-pass-pass-transport.aspx
Zone 1-3 (basically the entire city except for the airport) for three days will cover you for buses, trains, trams and metro.
EDIT Zone 1 - 3 covers everything...