Budapest is the capital and the most populous city of Hungary, and one of the largest cities in the EU. "Budapest", is the combination of the cities Buda and Pest, which, together with Óbuda, united in 1873.
At least 3 nights are required to get a good overview and be inspired by the city.
The Hop on Hop off tours are highly recommended in this city and metro can be used for more explorations. Boat trips on the Danube are also very nice.