I've experimented with Intercooler, and it's a nice intermediate step between just enhancing a site with some jquery or vanilla JS and full-fledged SPAs.

This is almost a general theme in webdev, the middle ground is often left out, with things accumulating either on the complex or purposely minimalistic. Rails/Sinatra, Flask/Django, React/Angular... (and yes, I'm aware that it's slightly ironic that RoR is now my "monolith" end of things, given how light it was back when we were doing horrible things with Struts and J2EE)