Just some ideas:
The first thing that pops into my mind is an Unseelie Fey convincing or controlling animals into attacking the group. I can easily see an Unseelie Dyrad siccing a pack of wolves on a group for giggles, or because they are enroaching on her teritory.
A grig lures a player high into a tree, then uses it's fiddle to cause them to fall due to the Irresistable Dance.
A pre-weakend board breaks in a bridge mid-crossing, plunging a player into the waiting grasp of a Glastig.
A Pixie uses its Lesser Confusion on a player when the group is trying to sneak past the cave of a large, angry bear.
A Nymph lives in a pool know for its beautiful water lilies tries to lure in the young men who visit it to find a flower for their lover, purposfully blinding them so that she can drown them and fertilize her lilies.
Three Satyrs get their entertainment by using the leaders pipes to cause fear in travellers, then chase and headbut them, stealing and occasionally kidnapping if they can, before running off.
Joystealers shadow the party, using their incorpreality, DR and Fly speed to avoid harm. They try to creep out the party and throw them off whenever they can. If they see an opprotunity, they will also go for a few points of Charisma drain. Lets see your players want to sleep when one of these can float through their tent wall at any time.
At night Red Caps bombard the players camp with slings and run around slicing at their stuff. Come morning the players are unharmed, but their camp is ruined, littered with rocks and covered in iron bootprints.

The one thing to keep in mind is that normal fey pranks and tricks can be malicious because they don't really understand the consequeneces, or are apathetic as long as they get their way. Unseelie are apathetic about it at best. Most know exactly what they are doing, and just don't care about the victim, and the worst ones do it because it causes suffering.