Hi and welcome to the DLRP Forums!
Congratulations on your trip to Paris. I'm sure you'll have a great time!
To get to Disney from the city, you should take the local train "RER" line A in the direction "Marne La Vallee - Chessy" (MLV Chessy) and get off at the final stop. The journey will take about 30-40 minutes and the MLV Chessy train station is right at the entrance of
Disneyland Park, Walt Disney Studios Park and Disney Village. It couldn't be any more convenient

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Wednesday 3rd October should not be too busy despite the fact that some French schools close early on Wednesdays, and it's German national holiday (which won't affect crowds too much).
I suggest that if (and only if) you can be there
before 10am (park opening), you should start with Walt Disney Studios Park. It's a small park that somewhat lacks in theming but still has its merits. You should get in line for Crush's Coaster right at park opening (and not a minute later) to avoid very long lines even on slow days due to the ridiculously low capacity of this rollercoaster.
After the ride, grab an early Fastpass for either Tower of Terror (identical to the Californian version) or Rock'n Rollercoaster (almost identical to the one in Florida).
In my opinion the only other must-do is the cinema show "Cinemagique" - so be sure to check the showtimes. If you enjoy character shows you might also wanna see Animagique.
"Stitch Live!" is the European Version of Turtle Talk, so you can skip it unless being a big Stitch fan (like myself). "Moteurs Action" and "Art of Disney Animation" and the "Studio Tram Tour" are very similar to WDW and they are not really good in my opinion.
Most would agree that "Armageddon Special Effects Show" is a waste of time.
2-4 hours after park opening you will have seen everything interesting in the Walt Disney Studios Park and be ready to hop over to Disneyland Park, which is a very beautiful park with a lot of attractions.
In Disneyland Park, the best thing to do is to grab a Fastpass for Big Thunder Mountain first, because those will run out the fastest. Also, the Parisian Big Thunder Mountain is by far the best in the world.
As a fan of the American parks, the whole park will feel familiar but also different at the same time to you. Many attractions are similar but yet there are a lot of difference. Because of this, I highly recommend you do Pirates of the Caribbean and Phantom Manor (Haunted Mansion in Frontierland). They are fantastic versions of rides you know from the US.
If you're up to it, the Parisian Space Mountain is very different, too. The train gets blasted into space in a giant canon, and it goes upside down 3 times. But be warned: The ride is very rough, shaky, uncomfortable.
Star Tours is not in 3D yet, but if you're a fan you should regard this as the excellent opportunity to experience the old version once more. Every park but Paris has already implemented the new one.
If you have time, The Disneyland Railroad, It's a small World and Casey Jr. are very nice in Paris.
On top of the rides, Paris has a couple of walk-through "attractions" that don't exist anywhere else. It will only take a couple of minutes to experience these, but it can be quite nice to awe at the theming

. These are:
The arcades (covered walkways) in Mainstreet USA (including Statue of Liberty Exhibit in Liberty Arcade)
Passage Enchante d'Aladdin at the entrance of Adventureland
The caves on Adventure Isle
The Nautilus in Discoveryland (very nice!!)
Taniere du Dragon underneath the castle
The upstairs of the castle
But by far the most important advice I can give you is this:
Be sure to see the brand new fireworks spectacular "Disney Dreams" at night. Apart from fireworks, the main point is huge video projections on the castle and on water screens on the side.
You will easily be able to find yourself a good viewing spot right in front of the castle about 15 minutes before it starts. Do not miss! I repeat: Do not miss under no circumstances.
Okay I am sure I forgot something and I'm sure opinions vary, but this should give you enough info until someone else contributes to this thread

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Have a magical trip to Disneyland Paris!!