A toy that simply beeps randomly. Hide it and drive someone nuts!
This is a simple project that is easily built on a half breadboard. I did the programming with a cheap USBasp programmer ordered from aliexpress. There are several guides on the internet describing how to program the ATTiny85 with a USBasp programmer, just do a google search on usbasp attiny85. My tool chain includes gedit and avrdude. I learned most of what I needed to know to set this up in "Make: AVR Programming" by Elliott Williams (mostly chapter 2). That book focuses on the ATMega328P (the same chip used on the Arduino), but is easily adapted to use the ATTiny85. I highly recommend that book if you're ready to take the next step; from programming in the Arduino environment into programming micro controllers directly.
Piezo buzzer, passive
9v battery snap
5V voltage regulator
push button switch
2 330ohm resistors
The simple program and makefile are attached.