Set some landmarks in the domain, they don't have to be immediately obvious to the players, but should they make a counter-clockwise circuit relative to the landmark they end up discretely teleported to the area of another random landmark. They should only be able to notice if they make a good spot/perception check or if someone happened to have detect magic going at the time they completed the circuit.
Have sneaky fey swap out their rations for fey-created food. If the players fail to realize the change has been made and eat the unseelie court's food, they have to make will saves to attack the fey leader during any climactic confrontation: it's simply rude to attack your host after accepting their hospitality!
Woodcutters, hunters, trappers, and hermits from the surrounding woods get kidnapped and baleful polymorphed into rabbits and deer. The players might find that out too late if they go hunting to replace fey-swapped rations.
no, the fey may have misplaced your provisions and will try to offer you food. Then if you accept it you are in their debt. They can't just exchange it, that would be foul play. (anything not foul play, however, is fair game )