Use silent image to make a small rock look like a valuable gem. As long as the fey concentrates, it will stay that way (drop the spell either as soon as a character puts it in their bag or if they call the others over to look at it). Especially effective against a greedy, selfish character with a low Will save.
Find a small dead animal. Put said carcass into the quiver of a melee character or into some other seldom-used location. Wait for smell.
Coat the character's hair in honey as they sleep. Sticky and annoying to remove, and especially hilarious if they're a late sleeper and the ants set to work in the morning before they rise.
Silent image a rotting piece of fruit as a delicious apple and place it where either the players will find it or replace some food the player is carrying.
Coal dust in a light-haired character's brush.
A berry in someone's boot as they sleep. If they fail to check them, a squishing berry feels much like a big spider.
After saddling their mounts for the day but before they leave, unbuckle the saddle so it falls when they try to mount.
Coat their hands in pepper juice as they sleep. When they wake up, they'll rub their eyes and mace themselves.