From CDG take the RER B (it will be marked "direction Massy-Palaiseau") to Gare du Nord and transfer to the Metro (look for signs that say "correspondance." Take Metro line 5 (marked "direction Place d'Italie") and take it to the Republique stop. Your hotel is a 7 minute walk from the station. You will find that Paris is a physically very small city as major capital cities go. There are no major tourist sights that are more than a brief Metro or bus ride from any more or less central location such as the one you will be staying at. There is no need to decide what to see based on proximity. Edit: The price of the ticket will be 8.70 euros. There is no additional charge to transfer from the RER to the Metro. Just make sure you don't exit the station while looking for the Metro. Signs will warn you if you are about to leave the limits of the station (they read "Limite de validité des billets")

Two options : 1 - The cheapest : Take the train, RER B from Aeroport Charles de Gaulle 2 TGV direction Massy-Palaiseau up to Chatelet-Les Halles connection Take the tube line 4 from Les Halles direction Porte d'Orleans (General Leclerc) up to Montparnasse-Bienvenue Count about 1 hour, buy a ticket "origine destination" valid zones 1 to 5, it costs 9,25€ one way. http://www.ratp.fr/ 2 - The easiest : Take the Air France coach, line 4, it goes directly to Gare Montparnasse, count about 1 hour. Prices are cheaper if you buy a return ticket in advance on internet : Prices if you buy on internet : 1 way = 15€ / return = 25€ Prices if you pay to the driver : 1 way = 16,50€ / return = 26,50€ http://www.lescarsairfrance.com/en/tickets-and-prices.html http://www.aeroportsdeparis.fr/ADP/en-GB/Passagers/Home/

See the links below, there are plans, pictures and all the explanations. The easiest is to take the Air France bus, line 4, it goes to Montparnasse train station, directly, without stops. By train, return ticket 18,50€ From the stop AEROPORT CDG 2 TGV Take the RER B Direction MASSY PALAISEAU to ST MICHEL Notre Dame RER B ( Paris) Walk 3min. Connections From the stop Saint-Michel Take the métro 4 Direction PORTE D'ORLEANS to Montparnasse-Bienvenue http://www.aeroportsdeparis.fr/ADP/en-GB/Passagers/Access-maps-car-parks/Paris-CDG/maps/ http://parisbytrain.com/charles-de-gaulle-airport-cdg-to-paris-by-train/ http://www.transilien.com/web/site/accueil/guide-du-voyageur/Informations-touristiques/lang/en http://www.transilien.com/ http://www.transilien.com/web/webdav/site/transilien/shared/documents/plans/pdf/plan_metro.pdf The easiest, though a bit more expensive, is to take the Air France bus, line 4, it goes straight from CDG airport to Montparnasse train station. Return ticket 26€ http://www.lescarsairfrance.com/fileadmin/documents/accueil/L4_plan.pdf http://www.lescarsairfrance.com/en.html http://www.lescarsairfrance.com/en/tickets-and-prices.html