CodeRunner <https://coderunnerapp.com> is a really useful utility app for Mac, a GUI scratchpad editor for writing and running code in dozens of languages, from Python and Bash to C/C++. It's not free or open-source, but IMHO it's well worth the $15. I use it a lot.
I put together a Nim plugin for it. Mostly I just needed to write a little shell-script to invoke the Nim compiler. The syntax-coloring library the app uses already supports Nim, but there's no syntax-aware indentation or autocomplete. Still, I find it really useful for trying out snippets as I learn the language.
To install: