If you want something simpler and more building-block, micro-framework style in Elixir, you'll want to take a look at Raxx[0]. You could also just build atop Plug itself, though Phoenix really is just a whole lot of Plug.
You might also be interested in Raxx.Kit, A project generator that gets your project started as quick as if you were using phoenix https://github.com/crowdhailer/raxx_kit