Its hard to be inconspicuous without an accomplice, though. You can't really juggle the clipboard, pencil, and water bottle with just two hands... Maybe if you gain an incredibly amount of fine motor skill with your toes...

Or grow a third arm.

 

Here's a nasty one. Because most people don't know how to work their own appliances (TV, VCR, etc.), you can pull nasty tricks. Simplest is to merely unplug something and see what happens. Otherwise, you can mess with the settings so it doesn't work and just watch the blood pressure rise! And if you really want to be mean, rewire it (can't just hit a button to fix, there's a power light so it looks like its on, and nobody will think to check wires because who would have possibly messed with those?)

 

Alternatively, tie fishing wire (or some other sturdy, hard-to-see material) across doorways, hallways, or whatever. Not ankle height, don't want to really hurt anybody. I've tied doors shut and watched as people get mad trying to open it up, and I've also strung it at chest and thigh height to get some funny faces when they run into it.

Something you can do in a college dorm when your RA is gone:

 

Take a rope and tie two doors together. This works best when the people who live in the rooms are in there. I did this today. I'll get the pictures/video loaded on the internet so everyone can see them. It was fun until the RA came around the corner and started yelling. Dick. I hate him when he's being the RA. The rest of the time he's alright.

 

EDIT: Here you go guys.

http://i74.photobucket.com/albums/i248/Mitth_raw_nuruodo/Tightentherope.jpg

http://i74.photobucket.com/albums/i248/Mitth_raw_nuruodo/Themedianoftheprank.jpg

http://i74.photobucket.com/albums/i248/Mitth_raw_nuruodo/Screwingw.jpg

http://i74.photobucket.com/albums/i248/Mitth_raw_nuruodo/JoetheMastermind.jpg

http://i74.photobucket.com/albums/i248/Mitth_raw_nuruodo/Comeonout.jpg

http://i74.photobucket.com/albums/i248/Mitth_raw_nuruodo/Checkinganotherlengthofrope.jpg