At this stage, you already know all the individual techniques used for this example. It's simply a matter of combining what you've learned so far to achieve the effect you want.
We'll try to illustrate a bit of everything with one sweet prank: you will prepare your Pi at home, take it over to your friend's house, and sneakily hook it up with the living room TV. In the middle of the night, the TV will turn itself on and a creepy video of your choice will start to play. This freaky incident might repeat itself a couple of times during the night, or we could take the prank to phase two: whenever someone walks into the room, their presence is detected and the video is played.
Let's start prepping the Pi! We will assume that no network connection is available at your friend's house, so we'll have to create a new ~/autorun.sh script to perform our prank, together with an at timer in /etc/rc.local that starts counting down when the Pi is plugged...