My most recent ex-sloot always liked those Garmin GPS/Heart Rate monitor ones. It would measure elevation, speed, distance, heart rate (you could strap some thing to your chest or stomach for more accurate heart rate monitoring). It could export data into all these funky graphs that would eventually show you trends based on weather, time of day, hills or flat, indoor and outdoor, etc.
I also see that my sister currently wears some Nike wristband/watch 24/7 now. It must serve some function because they made it all see through and theirs circuitry up in thurr. It's like straight out of the FUTURE, bruh's.
Me personally, I just like those old Digimon Pedometers where it'd measure the # of steps I took to help warm my pet, hatch him from the egg, groom him, feed him, etc.