I built a guitar stomp box using a Raspberry Pi Zero to trigger samples with a foot switch. Runs on 9 volts and has a very bright OLED display so I can still see what it's doing when I'm playing in a dark venue.
It's basically a Pi, a buck regulator, a tiny COB USB audio interface and an SSD1306-based 128x32 display. Removed all connectors and replaced them with soldered wires so it packs neatly into a tiny box.
The software is all custom and written in C, using Jack for low latency audio and my own driver for the display (Adafruit makes a Python driver but it eats half of your CPU and is not optimized for i2c bandwidth or high framerates).