It can be a challenge when you’re not using Rails to find gems which integrate well. Here are some solid options you can use in any Ruby web application.
Roda's stated goals are simplicity, reliability, extensibility, and performance. Those are the very reasons why I have become such a Roda stan. It's so malleable, you can take it in any number of directions in terms of architecture—particularly on the view side which is where my primary interest lies.
Use Ruby to transform incoming data and generate reusable markup, and use web components to encapsulate frontend styling and behavior. Build discrete building blocks of UI while collapsing mental models. Chef's kiss!