I can't imagine why you aren't flying open jaw but what is done is done.
You need to stay AT not near the airport. There are several hotels right at the airport; the Ibis and Novotel are budget hotels. If you want to go to Sacre Coeur you could just take a taxi; it will cost you 50 Euro and be quick. Find a walking tour guide (book) so you can map out your stroll -- you don't IMHO want to eat near Sacre Couer itself. Alas my favorite restaurant in Montmartre has closed so I will hope someone else has good recommendations -- or look at the Trip Advisor reviews for Montmartre restaurants. There are lots of awful tourist trappy restaurants in Paris and in areas like this especially so you will want to identify and perhaps make a reservation. If you don't do that, you might try one of the places on Abbesses.
If you want to save money and take the train, you can take the RER to Gare du Nord and then take the #2 line from La Chapelle (it is reached by following signs in the Nord train station; you use the same ticket) The two goes along the base of Montmartre. You can get off at Anvers and walk up Rue Steinkirk (sp?) and then take the funnicular up the hill or walk up the stairs. Or you could get off at Blanche two stops further down and walk up Rue Lepic to Abbesses and then from there wander up to Sacre Coeur which is a pretty walk in Amelie territory; you could eat at the restaurant in the film which is about half way up Rue Lepic or at one of the places on Abbesses. You could also take the little tourist train up the hill from there and go up to Sacre Coeur. (I think it starts at Pigalle stop) Frankly the Anvers entry to Montmartre is quite hideous and filled with con men running 3 card monte games, pickpockets and tacky souvenir shops -- going up Lepic or using the little train would be more charming.
My favorite route is to go to Lamarck Caulaincourt (you would have to transfer to line 12 on the metro from line 2 at Pigalle, still on the same ticket) You exit the metro and turn right and walk up the gradual incline to Rue Caulaincourt turn right and then at Junot across the street on the left up a block or two you cross over and walk up Junot. This takes you through some pretty parts of Monmartre. Towards the top of that hill is the little square with the sculpture of the man walking through the wall is -- an old French short story. If you go left and down hill here you come to the Vineyard and can turn right and walk up through very pretty houses to the back of Sacre Coeur. If you keep going straight and don't make that left, you come to Place du Tertre the Artists Square (fake artist tourist trap nightmare as far as I am concerned) and once you walk through the tourist tat you come to Sacre Coeur. This is a route that will give you a taste of the charming parts of Montmartre and there are cafes and restaurants along the way.
Be sure you are pickpocket proof with passports and cards and most money under your clothes in a money belt. This is pickpocket heaven -- particularly on the train, in crowded areas like Place du Tertre, and near Sacre Coeur.
You can easily take the #2 line at the base of Montmartre back to Gare du Nord (la Chapelle stop) to catch the RER B back to your hotel. When we did this the Novotel was literally on top of the RER line. If you drop your bags at a room here the train station is literally downstairs; buy tickets to and from and you are all set.
have fun
