The KRI Pasopati submarine monument is one of Surabaya's most unusual attractions: a fully intact 1960s Soviet submarine that visitors can climb through. The vessel is displayed in a public park in the south of the city, cut open along its length so you can walk through the cramped engine room, torpedo room, and crew quarters.
It gives a visceral sense of what life was like aboard a Cold War submarine — everything is startlingly small and functional, with instruments and equipment intact. The park around it has a fairground atmosphere, and the contrast between Soviet-era military hardware and Indonesian families out for a Sunday afternoon is very Surabaya. Entry is cheap; find it in Taman Hiburan Rakyat on Jalan Pemuda.