Photo: Ванилица, CC BY-SA 4.0 via Wikimedia Commons
This is in 3 buildings. Old Museum is a general display of the history and culture of the former Yugoslavia. 25 May Museum is an oh-so-Sixties building named for the official birthdate of Josip Broz Tito (1892-1980). It displays presents given to him during his 27-year presidency: he amassed quite a collection of youth relay batons. House of Flowers is his mausoleum.
The Museum of Yugoslavia is a public history museum in Belgrade, Serbia dedicated to the history of Yugoslavia and the life of Josip Broz Tito. Tito and his wife Jovanka are buried on the museum's grounds in the House of Flowers.
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Hram Svetog Save / Храм Светог Саве
The largest church in Serbia and one of the largest Orthodox churches in the world, under construction since 1935 and still not entirely finished. It's described as a temple (Hram) to distinguish it from the smaller St Sava Church (Tsrkva) adjacent. The crypt (downstairs from the main entrance) is an extensive, bright and polished area with walls covered with modern iconography, so it looks more like a metro interchange for Orthodox saints than a religious building. Services are held regularly here and in the Church, which you can also visit.
This operated from July 1941 to October 1944 under Gestapo leadership; at least 3849 perished here. It's now a museum.
Краљевски двор
Residence of the royal family of Serbia. You can only visit on a guided tour (in English or Serbian), booked through the Tourist Information Offices. The tour takes in the White Palace, with a large art collection.
Nikola Tesla (Никола Тесла, 1856-1943) made huge contributions to electric engineering, pioneering alternating current (enabling long-distance high-energy transfer), radio (underpinning today's mobile comms) and AC motors as used in household and office machinery. Half of this small museum is dedicated to Tesla's personal effects, while the other half contains models of his inventions. English-speaking tours start at the full hour and include demonstrations. Long lines build up at peak hours.