One of my favorite things to do on a Paris evening is catch a concert in one of the historic churches. The best way to find out about everything going on in Paris in a particular week is to pick up a copy of Pariscope, a small inexpensive magazine that comes out on Wednesdays and can be found at any newsstand. You can also find most of the information online.
Another way to find out what’s on at the various venues is by looking at the posters around town. If you’re keeping an eye out for them, it won’t take long before you come across a bunch. They are generally kept very up-to-date.