in looking for an interesting project for the digispark, I settled on a wearable heartrate monitor.
The microcontroller will be worn at the wrist. The photo here is a Teenys 3, but I hope to use the digispark when it arrives in the mail since its much smaller.
An LED + light meter will be worn on the fingertip.
The data will either go onto an SD card or use a Bluetooth4 radio to send it to a cell phone.