Nothing is simpler than finding the opportunity to go sightseeing by bus in Budapest nowadays, if you happen to be in the mood for it. There is a wide range of sightseeing busses touring around all the time, in various colours, with different roofs or mostly double-deckers, sometimes even floating on the Danube. However, this is not a recent idea as the first travel agency founded at the beginning of the last century, IBUSZ, operated sightseeing busses (or cars) with sunroof already in the 1930s, for between WWI and WWII the number of tourists visiting Hungary, mainly Budapest (see photos of the link) strongly increased. After the war blue-white striped, converted IKARUS busses were touring in town, operated by BKV, with departure from Roosevelt (soon to be renamed to Széchenyi István) square. Above clip shows a converted bus in the Sixties, arriving at the end to Vörösmarty square.
Vörösmarty tér a '60-as években / Vörösmarty square in the Sixties: