There’s a wealth of retro machines out there that cater to this. A sample: https://www.olimex.com/Products/Retro-Computers/Neo6502/open...

https://www.olimex.com/Products/Retro-Computers/AgonLight2/o...

Or with keyboards: https://wildbitscomputing.com/

https://www.kickstarter.com/projects/spectrumnext/zx-spectru...

Or you can go mobile: https://www.clockworkpi.com/product-page/picocalc

https://andywarburton.co.uk/post/gr3ml1n-a-compact-handheld-...

And if you want a real challenge, the one euro computer:

https://www.olimex.com/Products/Retro-Computers/RVPC/open-so...