I recently purchased this Keyboard FeatherWing that compliments the Particle Boron nicely. Since its based on the Feather form-factor, you could use it with any compatible controller.
I’m currently using it as a test/debug console. But there are so many possibilities!
It is currently out of stock on Tindie, but Pimoroni is still carrying it.
Thanks to @RWB for the idea to cross post this here.