I'm having flashbacks of this old cartoon for some reason and I strangely want to watch it again. In the beginning I remember there being a boy in his room who hears a parade. I think he has a blimp hanging up in his room and sees an identical one flying outside his window. They take him away to another world where he is given a key to the city and is told not to unlock a certain giant door. He ends up getting curious and unlocks the door. A kind of darkness escapes and starts to take over the "kingdom?". Also, I kind of remember him flying around on a carpet or something... very faded memory of this anime... Hopefully this is enough info to strike someone's memory.

Little Nemo, Adventures in Slumberland