The Psion Series 3 handhelds: https://en.wikipedia.org/wiki/Psion_Series_3 A (tiny, but) usable, physical keyboard on a device that lasted _ages_, fit into a suit pocket and was eminently practical. The age of the PDA is long gone (heck, I'm typing this on an iPad mini), but we still haven't caught up with some of its best bits.
Of all places I stumbled on it at an op-shop - in ~2012, I think it was. The box looked a bit sad but the unit itself was literally new.
...and I can tell you that if you've ever read stories about how fragile they were I can confirm all of them. The entire LCD hinge assembly broke on mine after only a few months, and I had to thread wire into and around the (~1mm no. 2 plastic >.The hardware was, in short, terrible, but the OS was amazing.
I'm convinced Symbian EPOC was built by a bunch of unbelievably optimistic, unfazeable software developers or something - C++ hadn't even been standardized yet (wow much 1998) so the EPOC SDK is a mix of scary, disastrous and hilarious... but the OS and UI design were still, in spite of this, absolutely awesome.
My favorite feature was the fact that the UI had real windows, with titlebars. And dragging the titlebars would drag the entire window - not an outline, the whole window. It was amazing. ("Look!! I have a desktop in my pocket!!")
And then there was the programming language that came built in. I mean, they crammed a fully-functional physical keyboard into the thing, so I mean, duh, you add a programming language to it cuz that's what you do when you have a real keyboard.
OPL was equal parts confusing and awesome, but, because it was so BASIC-like I started hundreds of tiny projects that I never finished. There are some pretty amazing little applications out there written in OPL, but most things were done in C++.
Probably the coolest thing I did with OPL was discover that the system was fast enough to handle full-screen haptic scrolling - my stylus was starting to get old and scratch the LCD, and using my finger would have felt weird, so I never finished it - but it was pretty awesome to know that this bytecode-interpreted language was fast enough to handle SurfaceFlinger-esque full-screen content scrolling on a 640x240 LCD.
What did your 36MHz ARM7TDMI-based gadgets do?