Work for dragons, I've loved dragons for as long as I can remember :D
2. Spend a day with the Order of the Phoenix or Dumbledore's Army?
I got to spend time with them when reading OotP so I'll say the Order. It would be interesting learning advanced defensive magic and learning war tactics.
3. Visit Dumbledore's office or Snape's dungeon?
Dumbledore's office. It seems really interesting, I'd love to have conversations with the portraits and say hello to Fawkes and the Sorting Hat (that is, assuming I visit his office before Dumbledore's death).
4. Give an old school detention to Filch or Umbridge?
Umbridge, she would deserve it more.
5. Have an Invisibility Cloak or a Time Turner?
An Invisibility Cloak, I wouldn't want to step on a bug or something in the past and cause some horrible catastrophe!
I love doing these.
I'M SEEING BEN TOMORROW!!! WE%GLJK#HJIG$LI@!!!!!