The KORG microKORG is a great synthesizer and vocoder launched in 2002. It does not feature a USB port but you can use a interface to convert MIDI to USB and use it to play with an iPad Pro (4th generation) launched in 2020.
This is a setup that I use to compose or play in the bedroom in headphones. I try to be not have a lot of cables and to be easy to set it up and down.
MIDI to USB: I’m using a LiDiVi MIDI to USB interface to connect the IN and OUT MIDI ports from the KORG into a USB hub. If you are looking for a MIDI to USB interface this one is fine but has extremely bright, blinking and annoying LEDs I will probably cover them with black tape later. I suggest going with a more discreet interface (although I like the colorful cables).
USB hub: I’m using a QGeeM 8-in-1 USB hub. A USB hub is not really necessary as I could use a USB to USB-C converter and connect the MIDI to USB interface directly to the iPad and enjoy their 4 speakers. However I want to use headphones. I have tried Bluetooth headphones but they introduced a unbearable delay between pressing the keys and the sound.
It is optional but convenient to power the USB hub so it can keep the iPad charged. Unfortunately you need the MicroKorg using either a DC 9 Volts AC adapter or six AA batteries. I’m using batteries to avoid having yet another cable. I’m my experience the batteries last quite a while.
The MicroKorg is full-fledged synthesizer with the capabilities of understanding MIDI that make it capable to control softwares on a iPad.
Big buttons for octave shift up and down
Small form factor
While not used as a MIDI device you can still use it as an standalone instrument.
You have to use an external power source or batteries.
A personal preference but I find that you have to press the keys with a little bit too much force in order to get the full velocity.
I’m currently using it with the KORG Gadgets 2 app but it will work with software that accepts MIDI input.