Ulaanbaatar's main sights are concentrated in and around the city centre. Sükhbaatar Square is the obvious starting point — the Parliament building, the Genghis Khan colonnade, and several major museums are within walking distance. Most visitors can cover the central sights in a day or two, leaving time to organise trips into the countryside (which is the real reason to come to Mongolia).
The Gandan Monastery in the west is the essential religious site — a working monastery after decades of Soviet suppression, with a 26-metre gilded Avalokiteśvara that dwarfs everything around it. The Choijin Lama Temple, now a museum, was the only monastery the Soviets preserved intact, making it the finest repository of traditional Buddhist art in the country.
For views, take the stairs to the Zaisan Memorial in the southern hills — the socialist-realist mural is remarkable, and the panorama of UB against its surrounding mountains is excellent. The Naran Tuul (Black Market) in the southwestern part of the city is the best people-watching in UB and worth a visit even if you're buying nothing.