Pico Audio adds an I2S decoder and headphone and line-level outputs to Pico. Pico has a capable audio system that can output I2S via PIO. You can create buffers of data that are streamed to your attached device. At the time of writing, this is in the pico-extras repository (hsmag.cc/pico-extras) because, while it does work, there is ongoing development and the API may change. You can use the audio system to directly output audio using PWM, but you can get better quality using I2S, and the integrated headphone and line-level amps make this easy to use.
If you prefer visual output rather than audio, Pico Display gives a TFT screen, four buttons, and an RGB LED, all within the Pico footprint. The result is a compact, encapsulated display, perfect for adding a little visual output to your project.
For a bit more sparkle, Pico Scroll adds a 17×7 matrix of white LEDs to the top of the Pico. As the name suggests, this works well for scrolling text as well as displaying data or just generally shimmering. There are four buttons to get data in from the user to control the display, or perhaps play a game of Snake.
Of course, you may want more than one of these, and for that you’ll need an Omnibus (for two add-ons) or a Decker (for four add-ons). You can also use these to access all the pins when another add-on is on top, whether that’s for debugging or expanding your project.