Disneyland Paris and Studios in One Day

If you've been to WDW a few times, that makes things much easier - hitting both parks is totally feasible. Do you know whether you'll be visiting on a busy day? (You can tell how busy it'll be from Disney's price brackets - "Regular" or lower will be manageable.) I'll sketch a plan below based on that. But on a busier day, I'd recommend just focussing on DLP, and only venturing over to WDS later in the day if you get a chance.

If you're able to get to the resort at least an hour before park opening - and bear in mind security checks at DLP take place on entry to the *resort* rather than the individual parks - go straight to WDS. Chances are, they'll already be letting people in to queue for Crush's Coaster. If you can't get to there ahead of opening, you're probably best to skip this one - there's no FP, and queues are always the longest. It's a fun, unique ride, but not worth losing a massive chunk out of your day for.

Whether you've done Crush or not, next get a FP for Ratatouille. Return time should be soon, but use the hour to tick off anything else in the park you want to see. Tower of Terror is a clone of the Californian version, so up to you whether you want to do that. Armageddon is not great, but it is unique to Paris (and also not likely to be around much longer) so you might want to hit that for curiosity value. Rock'n'Roller is the same track as DHS - just slightly different scenery - but should be near enough walk-on at this time of day, if you fancy it. (The Studio Tram Tour has one scene unique compared to the DHS version, but it's definitely not worth sacrificing such a chunk of your day for.)

If you're into shows, Mickey and the Magician is one of Disney's best. Kind of similar to Mickey and the Magical Map at Disneyland if you've seen that, but unlike anything at WDW. If it's something that appeals and you can catch the first showing after your ride on Rat, I'd recommend it - but it's not worth hanging around in the Studios for an extra couple hours for.

After that, you should be done with the Studios, so head over the DLP. Go straight to Frontierland, where hopefully you should be able to get a Big Thunder Fastpass for a few hours later. I'd eat now - you'll be near Cowboy Cookout which is a) one of the few fairly decent quick service options in the park and b) shouldn't have enormous queues. (DLP is notorious for only opening a few of their quick services at a time, leading to some awful queues - but CC often avoida this curse as it's slightly hidden away.) You might as well sit and eat by this point.

Check the queue time for Hyperspace Mountain - if it looks reasonable, go and do it now. If not, don't worry - you might be able to get a FP after your BTM time, and the queue goes down later in the day anyway.

After that, it's really a case of doing what you fancy. For a WDW regular, I'd say don't miss Pirates, Indiana Jones (not the most pleasant experience, but unique so worth doing once), a wander through Adventure Isle, a walk through Les Mystères du Nautilus, a walk through the Main Street USA arcades, and a walk through the castle. (Whatever you do, do *not* miss the dragon under the castle! I've lost count of the number of first-time visitors who've said it was their favourite thing in the park.)

Star Tours is the same as DHS, but in French. (Usually. I believe you can ask for an English version and the cast member will switch the language if there's enough English folks in board.) It's also currently locked on the same destinations as DHS, so you'll know exactly what you're getting.

Phantom Manor will sadly be closed when you visit, although a trip on the Molly Brown will give you a good chance to take in the beautifully landscape surrounding area.

it's a small world is a matter of personal taste. Personally I think it's the world's-best version... But it is still small world, so up to you. If you walk past and the queue is stretching out of the sheltered section, come back later.

If you're there on a 10pm closing day, you'll probably find BTM will get down to the kind of waiting time where you can give it another go. It takes on a whole new character at night - I'd really recommend doing it again.

Illuminations is worth sticking around for, if you can. It's not Disney's best kiss goodnight show - it's similar to Happily Ever After, with maybe slightly less of a punch - but a good time all the same. Usual rules apply there - try not to be too far from the castle, avoid obstructions like trees etc.

As you'll notice, there really aren't too many must-do rides unique to DLP. The charm of the place for someone who's visited other Disney castle parks is in its atmosphere, and the rich detail they've crammed into so much of it - the walk-through attractions in particular. So yeah, hit the rides, but also take the time to soak the place itself up. You're going to have a good time no matter what you do.

Good grief, I need to get a life.

I did both parks in a day when I visited years ago, and while it was a very full day it was manageable. We started at the Studios park and did everything of concern there in the first couple hours, then headed over to Disneyland Park and essentially toured it counter-clockwise (other than crossing the park when needed to use a Fastpass). Ben's plan is a very good one to use and should get you to everything.

Based on my experience at WDW and DLP, I'd say the following are highest priority:

Disneyland:
-Alice's Curious Labyrinth
-Big Thunder Mountain (much better than WDW version)
-Buzz Lighyear Laser Blast (if you've only experienced WDW's version)
-Fantasyland Dark Rides (if you haven't done the Disneyland originals)
-Indiana Jones et le Temple du Peril
-Le Chateau de la Belle au Bois Dormant (and DO NOT miss the dragon in the dungeon)
-Pirates of the Caribbean (much better than WDW version)
-Star Wars Hyperspace Mountain: Rebel Mission

Studios:
-Armageddon Special Effects
-Crush's Coaster
-Mickey & the Magician
-Moteurs...Action! Stunt Show Spectacular (time permitting)
-Ratatouille
-RC Racer (if you're a roller coaster person and the line is reasonable)
-Rock 'n' Roller Coaster (same ride as WDW, but different effects)
-Tower of Terror (if you've only experienced WDW's version)

Other than the above, make sure you take your time when exploring the park and take in the differences. To me, it feels like an alternate reality Disney park...familiar, yet distinctly different. Between the two parks, there's more than enough to fill a full day, so you won't be left looking for things to do. Just try not to get caught up comparing everything to Florida...Disneyland Paris is solid, but it's not the Magic Kingdom.

Ben gave you a pretty solid plan. Definitely take the time to explore this park because there is so much to just discover and appreciate. For example:
-The Main Street Arcades
-The fort at the entrance to Frontierland (Legends of the Wild West?)
-The paths between Legends of the Wild West and Adventureland
-Le Passage Enchante d'Aladdin
-Adventure Isle and the fort sections in front of Pirates of the Carribean
-Random Restaurants all around the park with fantastic designs (Toad Hall, Auberge de Cendrillon, Fuente del Oro, to name a few...)
-Alice's Curious Laberynth
-The entirety of the castle
-The Nautilus
-Random other corners, dead ends, and nooks and crannies with random things

Fantasia Gelati can be a good bang for buck if you get their largest size.

Have Fun!

David did for you what I was going to do. That's to remind you that you will be in hands down the most beautiful of all the "castle" parks in Disney realm. As such - don't forget to take it all in. DisneySea is the most amazing of the Disney parks, but Disneyland Paris park is the most amazing looking of Disney's classic Magic Kingdom parks.

Some of the areas to not rush through are the land transition areas. Moving between lands here is a real treat.

Ben's advice regarding Crush and Ratatouille is solid. will also add that at least when we visited there was a single rider option for Ratatouille. We rode it twice, back to back in less than 20 minutes total. There is no option other than to wait in line for Crush. It was better than I expected it to be, but in hind sight, I wish we had used that time to get to Disneyland park sooner.

Have a great visit.

If you stay at a Disney Resort hotel or buy a Season pass for 259 euro you can get the two hours of Magic mornings. Unlike in Florida and CA. The park won't be crowded during those 2 hours and you can knock out lots of attractions.

I've been to every Disney park in the World and what I enjoy the most is always the unique attractions. In Paris that would be there version of Space Mountain, Ratatouille and Crush. Well now you can also throw in there version of Lights Motors Action.

Crush is worth the wait.

Enjoy your visit.

Thanks for all the excellent advice, I’m hoping to try and hit this on my last full day, and have decided to try and find a room near by or at the resort so that I could possibly do DLP the first day and just hit Studios the morning I check out and head to the Airport. Any suggestions for a really reasonable room rate? Even if off property but close enough for travel/walking?

There's lots of good options in Serris. If you stay in the North-east end (around the Hipark) it's only a twenty-minute walk to the resort, or you can catch the free shuttle bus. (The bus isn't terribly regular though - at times only one every half hour.)

The Hipark is decent, and there's a few other hotels and Airbnb properties around there too. You might find some cheaper options on the West side of Serris (around the Val d'Europe station) but you're looking at closer to an hour to walk from there. (You do however have the trains as an option for getting to the resort, or the aforementioned shuttle bus - and that would be more convenient if you're arriving with bulky luggage.)

I'd definitely recommend off-site rather than on-site though. The DLP hotels are way overpriced for what they are, and the walk from the cheapest ones is long enough that you might as well have stayed somewhere nicer off-site.