If you travel from the UK to Paris by Eurostar (through the Chunnel) you will arrive at Gare du Nord. This busy station is in a bad part of Paris but has a integral metro stop. The Department of Tourism also has a kiosk outside the station where you can buy transport passes and museum passes. We used the metro extensively and found it as easy as London's to figure out. The French don't include "North" and "South" on the direction boards, but they always post the North end-of-the-line terminal first, so it's self evident. The transport pass has zones like London. I'd only buy zones 1/2, however that won't get you to any of the surrounding airports. You'll have to buy a single trip ticket for that (usually on RER - light rail). The connection to Orly from Place Michel, is exceptional - it ends at "Anthony" and you walk across the platform for the Orly Connector - a privately owned light rail that runs from Anthony to Orly (both terminals). Total time about 35 minutes.