Budapest - Milano. Any good destinations or party cities between these two cities?

Obvious stops are Bratislava, Vienna (direct from Bud), cross the alp via Semmering pass to Venice or Verona then Milan.

Alternative Vienna to Munich then Brenner pass (again to Verona or Bologna) or even change at Trento to Milan.

If you happen to now just have finished Sziget in Bp-then all will be just minor happenings.

In WINTer the well known -more booze as ski- after/apres ski in Tirol is more or less on the way in between.

Via a quite different route-not the fastest, you can go via Ljubljana-then Trieste-Venezia, and that Ljub certainly has a very youthful and entertaining atmosfere-a kind of secret, not well known. That is Slovenija