A set of nerdy Christmas tree ornaments with a central 5V power bus.
These ornaments are great teaching/learning vehicles for budding hardware and software hackers.
See http://cassettepunk.com/blog/2015/12/24/nerdy-xmas-tree/ for details.
An ATTiny85-based "snowdrop"-type light strip (made with neopixels) with a piezo transducer for interactivity.
An ATTiny85-based ornament that periodically says "ho ho ho".