Hi Alexandra,
If you're not traveling much your last day besides going to CDG, I'd recommend you buy a single
Billet Ile-de-France from whichever Metro or RER station happens to be closest to you. You can use that single ticket for the entire trip, starting with the Metro if need be.
At a ticket window just ask for a "billet pour Aeroport Charles de Gaulle". If you happen to have 8,40€ in coins, you could use a
Metro ticket vending machine or a
Billeterie Ile-de-France at the gares (Gare du Nord, Gare de Lyon, etc.).
When I first arrived in Paris, I was lucky to have had my girlfriend help me get a
Carte Orange/Pass Navigo. With that little piece of plastic, I never felt lost, anywhere in the city. I would wander, without looking at a map, wherever my fancy took me, stopping at boulangeries (which are everywhere) for snacks whenever I got hungry (which was often

). When I was tired or ready for a change of scenery in a different arrondissement, I would hit the closest Metro station (and there are tons) and go.
This is a great city for just "solo exploring". You can see so much history in the more touristy areas and get a feel for local life by wandering off the beaten path.
Here are some things off the beaten path:
- Parc Buttes Chaumont in the 19th
- walking along Canal St. Martin in the 11th and 10th
- the "chemin vert" elevated walkway on an old aqueduct, starting behind Bastille Opera house
- rue Montogueil, just north of Chatelet Les Halles, beside St. Eustache cathedral. The pistachio gelato at Deliziefollie is off-the-hook. And Charles Chocolatier is secretly the best chocolate maker in Paris (keep that under your hat).
Ben