One quick and convenient tour of Paris would be to take the Roissybus from CdG to the Opera.
To understandn the itinerary below, you need a good map of Paris or you can follow it on Google maps.
From the Opera you can
- walk down bld des Capucines to Madeleine,
- then down rue Royale to place de la Concorde.
You can have a glimpse of the Champs Elysées on one side, and the Louvre on the other.
From Concorde you can take a short metro ride (line N°12 direction Mairie D'Issy) to rue du Bac.
Then walk
- eastward on bdl St Germain to St Germain des Près,
- then northward on rue Bonaparte to the river Seine.
Cross the river to Palais Royal and check out the sumptuous garden (tricky to find the entrance).
Walk northward on avenue de l'Opéra toward l'Opéra Garnier; you can't miss the latter.
Somewhere along the way, stop in a café that everyone else seems to love and have a croque monsieur and a coup de rouge.
I imagine you have about safely 5 hours in Paris. It may be time to take the Roissybus back to the airport...
With this itinerary, you have an easy bus into town and out of town.
You will have covered the Opera district, Madeleine, Concorde, St Germain, the river, Palais Royal. Mostly walking, very easy walking, most of the time on large boulevards, theoretically hard to get lost.