I'm building a fitness tracker specifically for rock climbers using RFID in addition to the regular fitness tracker sensors (accelerometer, altimeter, etc). It started as a project to teach me lower level embedded programming (I do web dev for my day job). Since then it's expanded and we're actually testing it at a local rock climbing gym. If you're curious you can see a demo video here https://www.youtube.com/watch?v=6gnEAeMDKt8