I read a lot here in Tripadvisor befor trying to get the best from my 7 hours at CDG. We arrived at 6:35 to CDG (we were supposed to arrive at 6:10) so we have 30 minutes less of our 7 hours. We passed through inmigration in 15 minutes, and then headed to the RER B train station. We payed the 9.75 euros for the ticket to Paris. We had to wait for the traint to starts its journey around 5 or 6 minutes. And then, almost 45 minutes to get to the Notre Dame station. There we changed to RER C train and traveled for another 10 minutes before getting to the /Champs de mars/Eiffel Tower station. From there, the tower is 200 meters away. We walked at the side of the Senna river enjoying the view and taking a lot of pictures, but not staying for more than two minutes on the sam place. We even had the time to enter to an automarit toilet. Then we got the Eiffel Tower in front of us, and Trocadero behind. We took a lot of pictures, possing, smiles, etc. We even had time for buying iron replicas under the tower for a couple of Euros. The time was 9:00 a.m.
After that we walked to Champs de Mars, getting away from the tower, and headed to the Militar School. Just enough to be able to take the picture of the whole tower. Then, we returned to the train station, but this time we walked across the streets and not at the side of the river.
We took the same RER C train and headed to Notre Dame. We arrived there at 9:32am. The cathedral one block from the station so we walked and took pictures of the Senna again. At the church we took pictures outside and enjoyed the majestuosity of the monument. Then we entered (free of charge) the church and we took many pictures even in front of the altar. After that we went out of the church, we went downstairs to the river, sat there, and took many pictures again. The time was: 10:00am.
After that we decided to return to the airport. We took the RER B traing again, but we got the wrong train and had to get out of it at the Sevran Livry station. Time was 11:00 a.m. There we entered in panic, but after buying our ticket we had to return to the Aulnay station and then we headed to CDG on RER B3. Time was 11:40 a.m.
We got into inmigration and then we had plenty of time for shopping at the free duty shops. Time was 12:15
Our plane left at 2:00 p.m. Last boarding time was 1:00 p.m.
We did it! We visited Paris and with just 20 euros! I guess that if we had one hour more, we had been able to visit the front of the louvre museum or even the Arch!
We hope this info helps and encourages others to use wisely the 7 hours at CDG.