Catherine the Great liked the original Raphael's Loggia in Vatican City so much that she sent many painters there for 7 years to copy the entire thing. (Tons of other art too.)
A pedestrian-only island-park that included many hand-dug lakes and a palace
Built where Tsar Alexander II was assassinated, it is one of the few churches that looks Russian in St Petersburg; most of the others were designed by Italian architects.
The largest museum devoted to a single Soviet figure other than Lenin, Kirov's cushy home has been preserved since he was assassinated as Leningrad Party Secretary in 1934, plus his office was moved into a room next door. He really liked to hunt: there were more mounted animals than there were Stalin portraits.
I thought this might be a mothballed Soviet relic as was the Kirov house we saw in Novosibirsk, but it seems actually quite well attended by Russians.