I think Atari 2600 programming is super fun. It's 6502 assembly, which is pretty simple and fun on it's own, but mixed with unique, very limited hardware makes it even more interesting. You can do it online [1], but using DASM [2] and Stella and it's debugger [3] is even better. And if you ever want to run it on real harware, it's pretty cheap to make it happen.
[1] https://8bitworkshop.com/v3.4.2/?platform=vcs&file=examples%...
[2] https://dasm-assembler.github.io/
[3] https://stella-emu.github.io/