Stunning Mosaics
This tiny museum/church is fabulous. The mosaics are enormous and intricate, using rich colors to depict religious scenes on all walls. The floors are gorgeous too, with thinly laid geometric patterns of Italian marble. Not far from Nevskii Prospekt, the church won't take you away from nearly any tourist itinerary.
It is definitely possible to join surreptitiously onto a tour group here, particularly if you speak foreign languages. But at any rate, a tour guide is recommended, because they will help you truly appreciate the scenes that you see.