for those one-off times I want to just jump into a shell with, say, ranger available, it's as simple as
$ nix shell nixpkgs#ranger
if I am starting up a project that needs a tool, I just throw together a simple shell.nix in a folder for the project and run
$ nix shell
from that folder. there's ways to set it up so you automatically switch to a shell with the specified packages (iirc with direnv just like you'd do it for python venvs) but I haven't felt the need yet.
https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3...
https://nixos.wiki/wiki/Development_environment_with_nix-she...