Hello!
If you're staying offsite, I recommend staying in one of the partner hotels (you can find the full list in the Disneyland Paris website, they list their own hotels first, followed by the partner ones). They're cheaper, you can still get a package with the park tickets (and maybe breakfast), and they all have big (free) car parks and a shuttle bus to the parks. Otherwise you can park in the Disneyland carpark (20 euros), or the Disney Village (you pay by the hour, maximum is 24 EUR per day). The Disney Village one is actually much closer to the parks entrances.
When are you thinking of going?
The 25th anniversary celebrations are starting end of March: new parade, new shows, and new night show. If the new night show (with projections and special effects on the Castle) is anything like the current Dreams!, it will be unmissable. Every evening at park closing
Other unmissable rides are Ratatouille and Crush in WDS (as mentioned above, both with very long lines, so try to get into the park early, around 9:30 even if the official opening time is 10:00). If it's on, don't miss Mickey and the Magician (show in the Animagique Theatre in WDS). And maybe it's a bit old but I still love Cinemagique, also in WDS, also unique to Paris.
Back in Disneyland Park, Big Thunder Mountain is a must. Fastpass is available, but you need to get in early to get one. If you can, do it once during daytime and once at night time... Absolutely amazing at night!
Other great rides are Phantom Manor, Pirates (it's now closed until the summer for refurb), Buzz (someone told me that it's better than at WDW because you can move the guns more?). If the park gets very busy, you can go and explore Adventure Isle, or go around Alice's labyrinth. Don't miss the Gallery in the first floor of the Sleeping Beauty Castle, or the dragon beneath it!!
Think a bit in advance about meals... If you want a character meal, you must book it in advance. There are plenty of fast food places around (again, depending on when you're going, you'll find some of them closed), it's better to go for an early lunch (11:30 - 12:00) to avoid the crowds. There's a few buffets (again better to book them in advance) and some really nice (but expensive) table restaurants. You can see all menus (and prices) in the Disneyland Paris website.
There are a few apps for DLP. The official one from Disneyland Paris is not bad, you get a map, waiting times, and some basic info about rides and restaurants. I also have MagiPark, it only has the wait times (but in a much more concise way, easier to see), and you can make a list of your favourite rides to only see those times.
Hope this helps a bit! Have a great time