Sacre-Coeur is amazing, sitting as it does at the highest point in Paris on the hill of Montmartre.  As it sits at the very top of the hill, you can get great views of Paris and even better when you climb the dome of the basillica.  You should try to visit the crypts also as they are very interesting!

It is very easy to get to Sacre-Coeur by Metro then either walk up the hill (lots of steps) or take the tram.  The area at the bottom of the hill is not the best neighbourhood, but if you watch for pick pockets you will be fine.  But make sure to visit also the neighborood of Montmartre and not only the Basilica, you should start your walk at the Moulin Rouge, then go up the hill : take lepic street and then abesses street.  

Those two streets are pefect to discover french gastronomy : you don't have any supermarkets, only local grocrey stores. For example you have one bakery who has been elected as "the Best Parisian Bakery" in 2010 ( Le grenier à pain - 38 rue des Abbesses 75018 ) and which used to provide the Elysees Palace. You can also feel as a Parisian by drinking a coffee at le Café des Deux Moulins (15 rue lepic) : it's the café where Amélie was a waitress in the famous french movie Amélie

You will be in one of the most trendiest street of Paris, then you keep going up and you will see some windmills, vineyards, artists studios...

There is also a really nice and secret museum at the top of the hill where you can see all the work of artists who lived on the neighborood such as : Picasso, Utrillo, Toulouse Lautrec ...  

Make sure to see the view from the Basilica which is breathtaking, and be really carefull with pickpockets when you will go down the stairs to join the metro !