Most people's expectations of what a "basic website" should do have gone way up over time. Even as a programmer, I've fallen into the static site generator trap a few times. It's annoying to start a side project with a static site generator and then realise I want to add a small feature and suddenly I wish I'd just started with a simple Rails or PHP app. Nowadays, if I want a static site I just start with a folder of…
Jekyll is the most well known in Ruby space, but it's tailored to a specific niche - authoring a blog with Markdown or another lightweight markup language. You can certainly massage it into doing other things, but it's not that ergonomic as a general purpose static site generator.
If you want something that's easy to copy/paste into rails, a rack based static site generator like middleman is great because you can start writing with erb/haml and ActiveSupport from the very beginning.
If you're looking for the simplicity of handwriting HTML and CSS but you want some niceties like includes, partial templates, link helpers, nanoc is a good static site generator that's progressive. Start with plain HTML/CSS, only add additional features as you need them.