The µA723 voltage regulator is a classic from 1967, and was celebrated in the Boldport Club “Widlar” (Project #21). It is a bit boring to just regulate voltage, but since the µA723 contains an error amplifier and voltage reference … yes, it is possible to make an oscillator instead. My version of the Widlar glows an orange LED…
As always, all notes, schematics and code are in the Little Electronics & Arduino Projects repo on GitHub