I used to love the playful nature of programming on the Commodore 64. I think it was the immediacy of the environment and the screen editor that made it feel that way.

Stuff like PICO-8 and TIC-80 are great and are _almost there_ in my opinion. If you took those and made their whole API available from their immediate modes, as well as make their immediate modes aware of the program you've typed in their code editors so you can call your program's defined functions from the immediate modes, I think you'd have something really special.