Arduino TV Annoyer
This DIY Arduino TV annoyer allows you to turn TVs on and off as you please, annoying anyone watching them. The step step instructions Instructables have the parts at $1.50 (if you already have Arduino and the tools), but it looks like the instructions were compiled a number of years ago so expect the price to be a little higher.
A Raspberry Pi Trojan Horse
You can disguise a Raspberry Pi device inside a standard power brick for a laptop. The device will then create an SSH encrypted tunnel that would enable an attacker to send and receive data to the target network.
Turn a Safety Flare Gun Into a Wireless Camera Launcher
If you are the outdoorsy type who goes sailing or camping, you probably use flare guns for personal safety. Flare guns are great if you are lost or need to alert someone of your position to get help. If you have any spare flare guns lying around, then this may be the perfect hacking project for you.
Turn a Raspberry Pi into Pen Testing Drop Box
For this hack, you can turn a humble power strip into a stealthy Pi-Powered packet sniffer. For this project, you need a Raspberry Pi, a surge protector, a low profile SD card adapter, some cables, electrical tape, and a soldering iron.