Something like this (excuse my crappy ASCII art):

[NixOS module]
\- [option-set services]
   |- [option-set redis]
   |  \- [option enable]
   |     \- [boolean true]
   \- [option-set nginx]
      \- [option enable]
         \- [boolean true]

would compile to something like this:

{ config, pkgs, lib, modulesPath, ... }: {
  services = {
    redis = {
      enable = true;
    };
    nginx = {
      enable = true;
    };
  };
}