The Russian Museum — Best collection we've seen of not only constructivist art (Malevich, Popova, Tatlin and others), but also of their contemporaries working from different premises in that brief golden age of post-October Revolution creativity.
Museum of Political History — The two houses (one of a ballerina favored by Nicholas II, the other of a rich merchant) that became Lenin's headquarters in Petrograd, now joined as this museum, are themselves well worth seeing. The exhibits are a detailed history of Russia and the USSR, and now simply "Russia" again, 1917 to the present. In Russian, but with booklets with English-language explanations. Lenin's Petrograd office was in the best corner of the merchant's house.
Constructivist architecture in a working-class district of the city, far off the tourist route, but we got to see the most important examples still standing. Details to come.