Horse traders can be bought, gained, drawn, discarded, revealed, set aside, returned to hand, trashed, topdecked, or played. I can't think of anything else that can happen to a card. If we restrict it to one turn then I can still do all that:
I play crossroads, black market, two copper, buy Fortress and gain Fortress and topdeck it with Watchtower. Now I play Watchtower and draw Fortress, Procession, Prince, Cellar, and two Scouts. Now there is nothing left in my deck. I discard Fortress with cellar and redraw it. Now I Procession the Fortress, play the Fortress twice, trash the Fortress and put it in my hand. Now I play Prince, reveal Fortress and set it aside with Prince.
That's a nice round 10 things.
Certainly the lose-track rule has not been invoked, but I don't know if you would consider the Fortress lost track of at any point. I don't think there is anything else you can do to a card in Dominion (hopefully there are more with Adventures!)
EDIT: oh wait there are more. Pass to the left, return to supply...