If you’d like to get into wearables, or simply want a tiny Arduino-compatible board, the SENSable THING, AKA pico-Platinchen, comes in the form of a 20mm disk. This disk is surrounded by six solder points around the circumference, as well as 6 more on the inside for various IO and control duties.
To go along with this external hardware, it’s fully-compatible with the Arduino UNO (though with fewer IO points), featuring an Atmel ATmega328P main processor. Along with this, it has a 9 degree of freedom orientation sensor, including a separate ARM Cortex processor just to run it. If that wasn’t enough, it has a WS2812B RGB LED built-in, so unlike your first Arduino project, where you just “blink” the onboard LED, you could also have it change color!
Everything runs on a common CR2032 battery, keeping the assembly in a tight package. As pointed out in the writeup, in addition to a wearable application you could put it on your keychain to take with you wherever you go!