The National Shrine of the Immaculate Conception is a great place to visit if you're a Catholic in DC. The place is enormous--it has a big cathedral type space upstairs with little chapels entirely surrounding the perimeter, dedicated to Saints and nations. The bottom level has a crypt church as well as more of the little chapels dedicated to Saints and nations. It's a work in progress--they're continually adding more to the Cathedral every time I go. They also have a very good Catholic book store, if you're looking for something in particular.
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