If you'd prefer to avoid the RER and see more of the city, you could take the Roissybus to the Opéra. Then hop on the 22 bus to the Trocadéo (via blvd Haussman and the Arc de Triomphe). Walk out to the terrace and take in the views of the tower. Then walk over the Pont d'Iéna, do your thing with the tower (pre-book tickets if you want to go up, it'll save you a wait).

Then you've got multiple options - including the Batobus which will allow you to take a one-way trip along the river. You can jump off at Notre Dame, have a walk around St Michel, then take the RER line B directly back to the airport.