Not cool, but definitely nerdy - I could count and add in binary when I was in my second year of primary school!
While in my 20's and doing a lot of machine code programming, I figured out how to convert binary to hex to decimal and back in my head. I also had much of the ASCII table memorized and won a lot of bar bets for free drinks (it was a nerdy bar). And I could look at a serial RS-232 feed from a terminal on an o'scope and read off the letters it was sending. Not too fast, mind you. But if it repeated for a second or so I could recognize the patterns.
Nearly 40 years later when my son was taking computer networking class at tech school, I showed him the mental trick on how to do the binary to hex to decimal conversion in his head, and he was able to scare the teacher with these same mental conversions. Nowadays you can get an app for that, but it's really cool to do it inside your head. Never did pick up any girls with this trick though...