Building a small quadcopter back in 2012 or so (before cheap toy/consumer drones were everywhere) was a fun project.
Back then, it involved an Arduino, the internals of a Wii MotionPlus (cost effective way to get the gyro sensors), a plywood frame, and the open-source 'MultiWii' code.
That first build never flew well, but soon afterwards it started to become much easier to build a very stable quadcopter, as all-in-one flight controller boards started to appear, along with more knowledge of which brushless motors, props, and ESCs worked well together.