Disneyland

There is a direct bus from CDG to Disneyland. Disneyland to Paris is an easy train trip on the RER A.

a day trip to Disneyland is certainly doable, provided you don't have hopes of doing every single thing at the park. One day was more than enough for me. It is an easy ride from the Gare de Lyon to Marne-la-Vallee station, followed by a short walk to Disney. We left Paris before ten a.m. and were back in Paris by six p.m. There are lots of posts about the particulars of Disneyland Paris.

bus from CDG to Disneyland

There might be, but I would just take the TGV (10 minute direct train right from inside the airport).

As noted it's easy to get to on the RER, roughly 40 minute ride depending on where you are staying in Paris, and the Disneyland stop is the last one so you can't miss your stop. If you don't want to mess with the train there is a shuttle bus that runs daily. https://www.disneylandparis.com/en-usd/tours/disneyland-paris-express

A day trip is certainly feasible but I suggest a couple of things. Definitely buy your park tickets ahead of time, don't wait until last minute. They use dynamic pricing so keep an eye out on your dates when they open up. Weekends are always more crowded and usually more expensive. Though you can buy an undated park ticket, you have to schedule to use it on a specific date, so if they're booked up on your choice, you're out of luck.

Spend some time planning out what you want to do ahead of time and decide if you want to try to go to one or both parks. With one day, I'd probably just focus on Disneyland proper, but it doesn't cost much much more to get tickets to both parks. They're very close together and it's easy to hop between them. I'd plan to arrive right when the gates open to maximize your day. You have to pass through security before entering the main street area, but it's pretty quick even on a busy day.

Definitely get the Disneyland Paris app for your phone. The app shows wait times for all the rides and also helps with navigation around the parks. If there are specific rides you want to get on, you can buy a priority pass to bypass the line (though some rides you can't do this, so double check). Rides like Crush's Coaster and Big Thunder Mountain regularly have wait times of 90 minutes in the regular line, and can still be 30 minutes even in the priority queue. If you don't mind not sitting on a ride with your travel partner, the single rider lines are always shorter though can still be a wait for popular rides. If you have any interest in meeting characters, many of them must be scheduled/reserved in advance. There are countless social media groups specific to this park if you want to really dive into the nitty gritty. If you are just going for the experience and don't have do-or-die activities, you can just go and wander around.

I recommend bringing your own food and snacks as the food is expensive and pretty terrible there. You can't bring in alcohol or glass bottled drinks, but most other food and beverage is fine. Just remember you have to take your bags on rides, so don't pack too much :)