I need ideas on the best way to drive By public transportation

If you are asking for the best way to get from Paris city center to CDG by public transportation, the answer is to take the RER train from Les Halles or Gard du Nord. If you are outside of walking distance from one of those locations, take the metro to one of those stations.

Practically speaking, use Google Maps the day of to determine the best route, since it may change due to metro closures or delays. Also be sure to leave extra time since it is not uncommon for the RER to be delayed.

Since a metro ride is included as part of the airport ticket (14 Euro) you can save on the metro fare on the way to the airport by buying your airport ticket and using it on the metro ride to the RER station. I usually use the IDF Mobilities app on my phone to purchase Paris transportation tickets and use my phone to go through the ticket gates.

Use the RATP app for directions on your travel dates for up to date mass transit information. Google Maps is great for the walking directions but it is not always up to date on route closures.
Mass transit is the better choice to go from the museum to the Opera. You do not have to go back to the airport to get into central paris from the museum. The RATP app will offer you possible routes. You do know that the walk from the hotel to the museum will be well over an hour, right?

Hyatt Place Hotel walk to Musée de l'Air et de l'Espace

I may be misunderstanding but does this mean that you are planning on walking between Hyatt Place Paris @ CDG (241 Rue de la Belle Étoile, 95700 Roissy-en-France, France) and Musée de l'Air et de l'Espace (3, esplanade de l’Air et de l’Espace 93350 Le Bourget)? Looking at the locations of both on a map, it looks to be over 4 miles between them. More exactly, 4.3 miles/6.9 km. I'm a walker but that seems like a lot with luggage - which you'll have if moving from there to Opera in central Paris.

Again, apologies if I'm misunderstanding.

I agree with Kate. Walking to the museum seems impractical and time consuming, especially if you plan to bring your luggage with you. And while the Museum website says they have storage lockers for luggage and large bags, there is a size limit. So check your sizes carefully. It would seem most practical to take a taxi from the hotel to the museum. If you bring your luggage with you, then you would use the RATP app to find the most efficient combination of bus and metro to get to your Paris destination. If you leave your luggage with the hotel, you would need to return there, then go to the airport to get the RER or a flat rate taxi into Paris.

It would seem most practical to take a taxi from the hotel to the
museum.

I agree, and even probably without luggage.

Walking to the museum seems impractical and time consuming,

I wonder if it's even possible ! The autoroutes weren't made for pedestrians

Thank you!
Oh dear, I didn’t see this problem. I misjudged the distance (I’m really rubbish at reading maps, unfortunately). I’ll have to plan for another taxi ride then. Hopefully we can leave our luggage at the museum.

What’s the best way to get from the museum to the opera house? A taxi could be quite expensive, couldn’t it? It’ll probably be rush hour then.

Here is information on how to get to the museum. Note the luggage restrictions.
https://www.museeairespace.fr/visiteurs/acces/

Speaking of luggage, if by "opera house" you mean Palais Garnier, you will need to check your luggage somewhere else if intending to visit the Palais.

https://www.operadeparis.fr/en/visits/palais-garnier
https://www.operadeparis.fr/en/useful-information/prepare-your-arrival/palais-garnier

If you want to get from the museum to either opera house, use the RATP app for mass transit instructions.
If you want to get from the hotel to the museum, use the RATP app for mass transit instructions.
With respect to your luggage, is it bigger than 55 cm x 35 cm x 20cm? If it is, it will not be accepted; please don’t hope — go ahead and measure the luggage.

Another solution, rather than booking the Hyatt Place Hotel, book a hotel by the museum and take a taxi there. Then you can leave the luggage at the hotel, and after visiting the museum, walk back to the hotel and retrieve your luggage and take the 152 bus to La Courneuve where you can transfer to the No 7 Metro line direct to Opera.

My Cabintrolley 55x39x20

I guess, now it will be to much problems for me. My Cabintrolley is so little and it have to be 4 cm smaller. That‘s horrible. Now, I will search for a night more in Paris and will cancle the Hyatt for our group. No visit to the museum. The rules are to hart for my opinion.

A big thank you to you all for your help.

Short review:

Here a few things I liked
- Musée de l'Air et de l'Espace ( I found in another forum the information, that they offer big boxes for the luggage). They did. Luggage stored and we spent a very nice and interesting time. Liked it very much ( info: it was a hot day. The restaurant was not air conditioned/ not working?)
- Hotel Saint-Pétersbourg Opéra, Paris - I loved it. Very clean, very nice room with super working aircondition, window to open possible, quiet, friendly and helpful staff. Good breakfast buffet.
- the best in my opinion was the evening at Jules Verne Restaurant Eiffeltower !!!! I loved the food, the great view and the staff was so lovely. They did a fantastic job.

@fjord I'm so glad this worked out and that you loved the Air Museum! Quite gem, although it is not an easy place to get to you public transportation!

Good for you Fjord for finding a way out to the museum and I am so glad you enjoyed it. Glad you found a hotel you liked too.

We used a taxi from Airport hotel to the museum. From the museum to our hotel near opera we used first a public bus, had to move in another bus for one stop to metro station. I guess it was metro station La Courneuve Line 17. The bus drivers were very friendly and helpful, so it was easy to find the way. The bus drivers didn‘t speak english, but they understood and showed me, where we had to move.

Very glad to hear that it all worked out for you!

@ skunklet1771 - yes, it is. I thought of a hidden treasure.

There are so many wonderful stories about these amazing model airplanes, especially from the earliest days of aviation. I highly recommend it to anyone who enjoys airplanes, balloons, and the like.