Pałac Przebendowskich, on ul. Solidarności (corner with ul. Bielanska) is thought by some to be haunted, as is the Cytadela. Also the Chopin Museum on ul. Tamka might be worth a look. If you like parapsychology, also go to the Nowy Miasto and visit the very old red brick church just off the Rynek Nowego Miasto. No ghosts but one of those places where people sometimes feel special - ul. Freta that leads to the Rynek is a bit like that too.