I saw this movie when I was a child, so it must be from the 80's. The boy is from the city, he is sent by his family, or travels with his family, to an old mansion, possibly owned by an older relative. Inside the mansion there is a girl ghost who meets the boy and the rest I remember is that they are dancing, and then kiss. Anyone knows what movie this is?

Try Little Ghost (http://www.imdb.com/title/tt0119550/reference)

Well, that is easy to answer!

It's "Child of Glass (http://en.wikipedia.org/wiki/Child_of_Glass)", which is based on the book "The Ghost Belonged to Me (http://en.wikipedia.org/wiki/The_Ghost_Belonged_to_Me)"!

This is it! Little Ghost. Thank you! This haunted me since my childhood haha, mystery solved.

You're welcome.