Forth and Lisp, easier with S9 than Guile with tons of modules where some Guile libs overlap with SRFI (global standards for every Scheme to follow). You can almost trace Lisp functions by hand.

Ditto with Forth dumping the memory, creating literal structures for numbers and whatnot. Also, the 'see' command among dumping literal memory bytes.

Being both a REPL helps a lot. But Forth gets into a lower level than S9 itself.