That's the nice thing about the newer chips (AVR, ESP8266, ESP32, STM32, etc). All those peripherals are already included on the SoC or the dev board. It is very easy to quickly upload the firmware through serial-USB, and start having fun.
My first CP/M computer was an Ampro Little Board 1A, sold along with a floppy carrying CP/M-2.2 + ZCPR3. I got a lot done on that machine, including a text-mode MIDI editor-sequencer -- the Z80 DART as set up could run the second serial channel (the first was CON to the terminal) at the MIDI baudrate and slave-interrupt to derive its MIDI Clocks from a sync-track recorded/played on a 4-track cassette machine through a Kansas City tape-modem.
WDC makes a few dev boards for the 6502, also available on tindie.
If you can't get one, and who can -- then the N-GO is a good alternative.
There are other clones out there, e.g. Sizif 512: https://github.com/UzixLS/zx-sizif-512
But for modern kit, readily available, probably the Agon Light:
https://www.thebyteattic.com/p/agon.html
E.g.
https://www.olimex.com/Products/Retro-Computers/AgonLight2/o...
I've also been eyeing their TI-92, 92II, 92Plus, and Voyage 200 series of calculators. They use the 68000 processor and have qwerty keyboards. It looks like there is a C compiler for it (http://tigcc.ticalc.org/), so it ought to be possible to do some interesting things, like this Fuzix OS, or a FORTH, LISP or Lua interpreter, or a terminal program over a serial link to a battery-powered ESP8266/ESP32 to the internet over WiFi.