Okay guys, this one shall be the most epic prank on the discussion so far...
When the PCs go to sleep, immediately ask the players to read aloud all the items in their backpacks for your personal reference. If they ask why, just smile and refuse to answer.
When the players awake, they see their bags, UPSIDE DOWN!!!
As the players go through their bag to see which possessions have been stolen, read aloud the following text, making sure to prolong the pauses:
"Much to your shock and horror... you discover... that ALL of the items you have placed in your bag... are now... ... ... STORED NEATLY!!!"
If you can create as much horror as possible in the way you say "stored neatly", you may get a player yelling "WHAT THE F&*#!!!"
See, thieves, players can handle. But beings that pack their possessions neatly, that's just creepy! It will mess with them, trying to work out who would pack their possessions neatly and then leave their bags upside down.
You could also have them wake up to find they are surrounded by a gang of spear wielding potatoes. Have the players roll initiative. Once each has had their combat round, explain that the potatoes are just normal potatoes with small spears attached.