An old calculator
When I was in middle school my parents got me a Sharp PC-1401 ( a slightly earlier version of this 1403) for Christmas. I had a Sharp EL-506 calculator, and the PC-1401/3 was an extension of that style but included a BASIC language interpreter and a full QWERTY keyboard. It was the precursor of the laptop, and I carried it around every day. I spent quite a bit of time writing basic routines to calculate astronomical ephemeris of various kinds. As you can imagine this behavior was quite the magnet for attention from the ladies.
It is a surprisingly easy to use interface, and it has terrific battery life. I used it for a long time before eventually moving onto the classic HP48. Of course now the watch I wear has 10,000 times the processing power, but for those times, it was something.
Nerd on.