You made sure to scream, "What was that!!?" immediately following the ghost's compliance with your requests, right? As if, after just talking to it and acknowledging it's presence and ability to alter the environment, you suddenly act as though it couldn't possibly have done that.

I'm sure that's a form of provocation in it's own right.

I think levitation would do it for me.  Levitation is good, I like levitation.  But obviously it would have to be non-hokey, full daylight levitation in an environment I knew wasn't tampered with.  If that exists.

One time I dreamed I *was* a ghost.  It was a very vivd dream and it was one of those dreams that seem to last for weeks.  Someone killed me, and I couldn't rest in peace until I got my revenge.  But I quickly found that nothing I could do, and I mean nothing, would let me communicate with anyone alive.  For a long time I hung around my parents' house trying to get them to know I was there, which was very frustrating.  Eventually I placed myself so that my dad reached right through my chest and he showed no sign of knowing anything was out of the ordinary at all.  That was discouraging, so I left.

After that I remember just wandering around a city.  Some time during my adventure I discovered that all objects have "ghost" analogues, and that a ghost could separate the two.  You just try to move something, and you move the "ghost" object, leaving the "real" object behind intact.  Eventually I also discovered that the two objects are so intrinsically one that separating them was like a violation against space-time or something, to the point that in an area where many ghost-object pairs were separated, even living humans became vaguely aware that something wasn't right, which manifested as a creepy feeling.  So even if everyone alive was completely oblivious to me, there was still this one way I could affect the world of the living. 

In time I met a female ghost about my own age.  Ghosts were very rare in my dream, and she's the only other one I met.  I told her about a theory I had where what we were doing with the separating objects had to be doing something to the fabric of the universe, and if we separated enough pairs in a small area, it might bend the rules enough for us to slip a message through in some limited fashion.  So we went to my parents' house and started separating everything we could find, and I think it worked.  I vaguely remember my parents being able to see me for a minute and telling them what happened.  After that I don't remember a thing.  I wish I could remember how it ended up.  I think the plot would make a decent movie.