A while ago, I decided that I wanted to build an analog modular synth. After doing some research, I found out that I didn't know all that much about analog audio electronics at all. So the best thing to do was to start from the very beginning and to build all kinds of simple oscillating circuits to get a grip on the basics.
When you google for simple oscillators, you inevitably come across the stepped tone generator aka the Atari Punk Console (APC). The circuit was first published in 1980 by Forrest M. Mims III and was later slightly altered and renamed Atari Punk Console by Kaustic Machines. They gave it that name because its sound resembles that of the old Atari video game consoles.
When I did build my first APC, I quickly discovered why it is so immensly popular. It is very cheap and easy to build and it produces a lot of weird and funky noises with very few parts.
The only thing that, imho, is missing in 90% of the available circuits online, is a way to add an external voltage control. It can be done by adding only two extra female Jack connectors to the two free pins on the 556 timer. WIth these two extra connectors, you can add an external oscillator, LFO,...
A 100K Logaritmic potentiometer or a 100K and 4K7 resistor
3 female jack connectors
A DC connector
A 9V powersource
A case. I used a plain and ready available project box. But google for Atari punk console and be amazed about all the weird and wonderful housings for this device. I am sure that you can come up with one of your own.