If you just need to put a few lights, sensors, or other intelligence into a project, this tiny Arduino Nano setup provides everything you need. It's powered by a (2) CR2032 battery pack, options can be selected via (4) DIP switches, and everything is held together with shrink wrap!
Here's the basic electrical layout, and details of how it's put together are available in the video. Code and a parts list are available on GitHub.
Once done, you can use it for simple tasks, like this PIR blinking thing!