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. 

Take the time to visit the artist square of Montmartre near the basilica, and the Salvador Dali museum. There are many restaurants and bars.